My POLINI THOR 250 Powered SKYMAX PPG Quad Project

Parts of this web site were last updated on November 6th, 2022; thanks for visiting! 

Go to the bottom of this page for the photo story of the THOR 250 Magneto's In-Flight Failure when low over rough desert back-country terrain, and the recovery.

Some of the most recent updates and modifications are now at the top of this page;other updates were added at the bottom.
Latest additions deal with a replacement electric starter, starter brush wire breakage repair techniques, and an (out of spec) carburetor replacement jet needle causing changes in the fuel mixture, requiring  re-jetting

This is my Polini THOR 250 powered PPG Quad, flying up to 15,029 feet ASL so far and flying cross-country flights of up to 67.4 miles.

ABOVE: This 2014 THOR 250 Engine now has a new PWK28 carburetor, an upgrade electric starter, and a new 145 cm diameter E-Props NG-D ground adjustable pitch propeller. My early impression of this high-tech propeller is that the thrust on takeoff is noticeably improved, allowing me to get up to flying speed and off the ground after a substantially shorter takeoff roll.

UPDATE: AXLES UPGRADED  TO 1-1/4" solid Fiberglass on July 26 2021. The photo above shows the new one piece front axle; details & more photos below, and towards the end of this page.

Photos just below show my PPG Quad just after landing. Note the set of four riser guide rings; the outer larger diameter set are for the wing's risers, while the inner set, mounted just behind the main riser guide rings, are used for the routing of the rescue parachute's extended risers. The rescue parachute is connected into separate low hang point webbing loops, with a separate set of twist-lock caribiners. Tip steering lines and handles are routed outside the main riser guide ring for free-running operation.

Below: Rear view. This 65" I.D. Prop guard cage has plenty of clearance for the 145mm adjustable pitch propeller; it's six upper cage support struts help this cage provide solid protection. These 1-1/4" thick solid fiberglass axles combined with the 15" wide smooth rib wheels handle the rough character of the natural short grass prairie well. The Buffalo Peaks on the western edge of South Park are seen in the distance.

Below: Left side view, showing the reserve parachute mounted with 3/4" cam buckle straps. Colorado's Mosquito Range with six 14'ers is seen in the background.


I had flown my 'Bluehawk' PPG trike since 2013- primarily flying in Arizona's deserts at roughly 1800' ASL. That trike is set up with a HIRTH F33 engine which is rated to produce up to 28HP at sea level. The Bluehawk trike is a heavier build on a Flexfoil Kitebuggy under-carriage, built with 3/4" heavy walled ChroMoly steel tubing, with a dry weight of roughly 220 pounds. For flying from my 'home field' elevation of 9,940 feet, where a normally aspirated engine (without a turbocharger) produces roughly 70% of it's sea level capability due to the reduced air density, I wanted more power / thrust, and I wanted to fly in a substantially lighter weight rig.

So I set about choosing a stronger engine and a lighter weight quad airframe. I bought one from From there, I've been going over and optimizing all of the details in an effort to come up with a highly reliable PPG quad rig for my flying purposes. From my own perspective, many things needed modifications; This page offers details of the modifications I've made as well as tuning tips, information sources, and parts options & sources.


Below: I fly with TWO mirrors on my quad; the upper mirror gives me a good view of how the wing is coming up off the ground as I power up to start my takeoff roll. The lower mirror gives me a good view of the center of the wing when it's overhead. Being able to see how the wing is initially inflating & coming up off the ground helps a lot on takeoffs.

The top (dark) instrument is the EGT meter just above the white Polini Ignition test / switch box. On the left are a Tiny Tach above and a coolant temperature gauge below. I fly with my Garmin GPS Map 64 attached with velcro to the  pack; inside I have survival & self-belay evac gear (in case of a emergency landing in heavily timbered country), as well as the wing storage bag, cell phone, 38 mile max  range GMRS radio, and other personal items.

Trailering for long distances & over rough roads is hard on the axles and structure of a PPG quad. Since most of the weight is on the rear axle, this is where adding a 'trailering support block' can minimize the stress on the rear axles & their mount tubes.  This one was designed to sit on top of my loading ramps and to support the axle carrier tubes. It has a 2x8 lower base plate, then two 2x4s stacked- all glued & screwed together. Two eye bolts allow attachment to each axle with cam buckle straps as seen in the photo below.


Thought I'd pass along an alert on my experience of 08-27-2019. The attached photos tell a lot of the story.

I've been flying a THOR250 engine on my PPG Quad since 2014.

I use an EGT meter to monitor / set jetting & carburetor adjustments, run 91 octane autogas (typically with 10% Ethanol) with LUCAS 2 cycle mix oil at 50:1,  with Stabil 360 also added, and 1/2 ounce of Lucas Octane booster added per 5 gallons. I have the thermostat installed, and on startup, I always warm the engine to a coolant temp of ~130 degrees F before running the engine up to full power. I then check full throttle RPM is about 7500 RPM, as intended, before taking this quad off the trailer & setting up to take off.

In May of 2018, I had pulled the cylinder and removed carbon from the piston and rings. The engine has run strongly since then.

Iíve been flying recently near my home in the central Colorado Rockies, in the South Park region, so Iím tuned for flight operations from a launch / landing site at an altitude of 10,000 feet ASL. (#120 main jet, two blade 56"  GSC Tech series prop with pitch set at 11.4 degrees) Peak engine RPM is at 7500 (relative to atmospheric conditions), with EGT peak readings between 440 C to 480 C when running at full throttle - well below max EGT reading recommendations. Run has been strong & consistent at these settings, with flight operations up to 14,600 feet ASL in the last month...  That is, until the morning of the 29th.

After takeoff & climb-out with normal running conditions, Iíd set the cruise control to ~7200 RPM. Wing trimmed in full reflex mode for cross country flying. At about 8 minutes into the flight, the RPMs began to drop off fairly rapidly, and then the engine stopped running. An attempted electric restart was unsuccessful- the engine would not turn over. I then reset trims for full slow / minimum sink and glided to a landing .8 miles away on reasonable open ground.

Once I brought the rig back home, I did a quick check of the carburetor, then pulled the spark plus to find that both of them had been hit with internal aluminum debris, battering the spark gap closed. That did not look good at all!

So I next dismounted the engine and took it into the shop. Fragments of aluminum piston skirt, etc. fell out of the exhaust port when I lowered the piston and shook them out. After removing the 4 head bolt nots, I lifted the cylinder & head off to find that the lower sections of the piston skirt had disintegrated, resulting in pieces of the cylinderís lower wall flanges to also be broken away.

Rod, wrist pin & bearing are still in good condition, showing minimal wear, and the connecting rod lower end bearing also is in good condition; this tells me that  lubrication has been good. This in-flight shutdown resulted from the failure / breakdown of the pistonís lower skirt after 164 hours of operation. Why would this occur?

The lower ring is still free & a good fit in it's groove; itís hard to tell about the upper ring after the debris was sucked up into the upper cylinder, but there appears to be some carbon buildup there since the cleaning in May of 2018. (This is engine # 2061, bought in Jan. of 2014)
I've ordered replacement 'Cylinder Set D.72" and have thoroughly flushed the crankcase; I'll be re-assembling this engine and starting the break-in procedure runs once the parts make it here from Italy - (they are on back order right now through ) I'm switching to AMSOIL Dominator 2 Cycle oil, mixed at 2.5% for initial break-in, then will run it at 50:1 mix after that.

TECH NOTE: In their service recommendations, Polini recommends de-carboning the piston and rings and decompression hole after 100 hours, and recommends replacing the piston, rings, wrist pin and bearing after 150 hours of operation. They also recommend replacing the full cylinder set after 300 hours.

The  early history of my Quad

While doing my research in late 2013, I came across James Weibe's excellent review of the POLINI THOR 250 engine, his installation in his BELITE ULTRA CUB, and the story of his cross-country flight from Kansas to Oshkosh, Wisconsin and back with only a couple of minor issues. (The link to this review is at the bottom of this page. )

With a power rating of up to 36.5 HP in a high reliability water cooled light weight engine, I decided I'd found the engine I was looking for.

The Water Cooled POLINI THOR 250 Dual Spark Engine

The Polini THOR 250 DUAL SPARK engine is provided with two spark plugs and two independent ignitions to ensure maximum security features in each flight condition. Liquid cooling system grants an extraordinary constancy of performance and more power at every RPM.

Polini THOR 250 DUAL SPARK engine has many more advanced solutions, including:

- The cylinder is die-cast in light alloy, it ensures a better thermal stability.

- The piston, gravity cast at high silicon content light alloy, reduces thermal expansion and the coupling slack. The design of the piston top is specific to optimize the compression ratio.

- It is provided with a balancing countershaft; this feature cancels the vibrations and guarantees such a driving comfort never felt during a flight and a longer life for the engine itself. The counter-rotating rollers decrease greatly the upsetting torque-steer for an extra comfort in flight.

- The centrifugal clutch is in oil bath with helicoid mechanical reduction.

- The electric starting is standard but to offer the maximum safety the engine is also equipped with manual starter with easy system thanks to the flash starter device that speeds up and simplifies the movements.

- It is equipped with the comfortable closed-circuit system for the recovery of fuel during transportation and the 12 V output for any use.

Small and compact THOR 250 DUAL SPARK engine has an extraordinary power to weight ratio.

Advanced technology and high performance of the new THOR 250 DUAL SPARK allow a better feeling of driving performance, making the flight more dynamic, responsive and sensitive, besides ensuring even greater stability.

The new THOR 250 DUAL SPARK engine adapts to different types of applications including one or two-seat trikes, powered hang gliders, one-seat small three-axis ultralight aircraft, and ULM motor-gliders

Polini THOR 250 DUAL SPARK is a 100% Made in Italy product.

Technical data Polini THOR 250

Engine: 2 stroke monocylinder, Liquid cooled

Displacement: 244 cc , Bore for stroke: 72 x 60

Power: 36 HP at 7500 R.P.M.

Cylinder: Aluminum with Gilnisil coating

Compression ratio: 11,5:1

Piston: Two chromium plated rings 1mm

Intake: Reed valve in the crankcase

Carburetor: PWK 28

Air filter: Air box

Ignition: Magneto and Electronic ignitions included on Dual Spark version

Battery charger included; Output power 80 W at 5500 RPM

Spark plug hood 5k Ohm resistance

Fuel type: Lead free petrol with 2% synthetic oil minimum ; 50:1 typically used after break-in

Gear reduction unit: Helical teeth in oil bath with 2,8 reduction ratio

Starting: (Electric starter optional) Pull start with self winding cable FLASH STARTER

Clutch: Centrifugal in oil bath

Muffler Expansion with oval silencer

Engine weight 18 Kg (19kg with electric starter) without radiator: =43# without radiator, ~54# complete installation

Propeller rotation: Clockwise.

THOR 250 Normal Operating Ranges

Water Temp 130 to 185 F; Max 194F

Cylinder Head Temp 130 to 185 F; Max 194F

EGT Operating Range Centigrade: 500C to 620C ; Max 650c

EGT Operating Range Farenheight: 932F to1148F ; Max 1202F

Best Efficiency EGT Operating Range: 1075F (~579C)  to 1148F (620 C)

Operating RPM Range ~1700 to 2200 RPM Idle, to 7500 Peak Power ; Red Line 8000 RPM

Fuel Efficiency ~3 litres/hr @ 5300 RPM ; ~8 litres/hr @ 7500 RPM (When jetted for sea level operation)


On June 7th, 201 I flew  a flight of 2 hours and 1 minute, covering 65.9 miles. I started from my launch area at about 9800 feet. I flew north & west, crossing Georgia Pass, flew north to pass over Swan Mountain just south of Lake Dillon, then flew back south over Hoosier Pass to return to land at my launch site. During this flight, I flew up to a maximum altitude of 12,905 feet ASL .  After landing, I measured the fuel used Engine RPM was up to 7450 RPMduring most of the climb, crossing over the continental divide twice on this cross-country flight. EGT stayed within the peak efficiency range. I used about 3.5 gallons of fuel on this flight.

Update JUNE 2018: I'm  flying the Mac Para Charger reflex wing; I have a #122 main jet installed. At full power & high RPM while climbing, I may use from 1.75 up to 2.0 gallons of fuel per hour. (Cruising at reduced RPM uses substantially less fuel.)

There is a good review at this PPG website:

The  Skymax  Quad

John Fetz had introduced me to Leon from Skycruiser Manufacturing while I was in Arizona in 2013. I had enjoyed meeting him & his wife. So after coming across a Youtube video of him test-flying one of the THOR 250 engines mounted on one of his PPG Quads, I gave him a call, and made a deal for him to do the installation of the THOR 250 Dual Spark engine onto a new SKYMAX Lowboy Quad.

Polini water cooled THOR 250

 Skymax Version

Also Available in Lowboy 

Now Using the Polini THOR 250 Engine which is water cooled for great durability 

Light Weight and Powerful

Above is a 2017 photo of Skycruiser's SKYMAX QUAD with the Polini THOR 250 engine. Contact Leon at Sky Cruiser Manufacturing:

FEATURES:Chrome-moly steel tubing used for the main Quad frame with an aluminum prop guard cage. Powder Coated ; 1" Solid Fiberglass Axles, fold down seat. Bare Quad weighs in at 58# before adding cage and motor installation. Low Attachment Points for easy inflation, good ground handling stability.


UPDATES: The photos and narrative below show some of the work I've done to fine-tune my PPG QUAD which I had ordered with it's water cooled POLINI THOR 250 Dual Spark engine installed. (The photo above is not one of  the Lowboy versions; it shows a different seat and fuel tank than mine; I like the extended fill neck fuel tank's looks, but it may be a lesser capacity.)

Above: My modified Skymax PPG Lowboy Quad with the Polini THOR 250 engine after many modifications as of August 11th, 2016.

In July of 2017 my rig's dry weight after modifications, ready for flying is checked at 135# with no fuel in the tank. Mounting the Apco Mayday Bi rescue parachute adds 9 pounds. Fully loaded with fuel and gear ready to fly, it's at roughly 180 pounds. With me in the pilot's seat with helmet and clothing, etc. my takeoff weight may be close to 355 pounds.

TECH NOTE: The minimum recommended operating temperature for this engine is at a water temp of 130 degrees F. While Polini now offers an optional 60 degree C (140 degrees F) thermostat (Polini part # 928830009 ) which mounts inside the cylinder head water hose fitting, it was not included with engines being shipped when I ordered my engine. With the rather large radiator, many flyers were reporting covering up to 2/3 of the radiator surface in order to get the operating water temperature to stay above 130 F degrees while flying. Adding the thermostat makes a lot of sense to me... flying with the engine below minimum temperature or flying with duct tape covering part of my radiator in hopes that I've covered the right amount of the radiator's cooling fin area doesn't work; I'm seeing H2O temperatures in flight that are much too low. UPDATE: I've now installed the thermostat- see details & photos below.


Above: The Thermostat kit parts

Above: Coolant Drain plug with red fiber gasket on bottom of the water pump. 8mm wrench. (Once a catch container is in place, opening the reservoir cap allows the coolant to flow out freely. Save & reuse the coolant- there's just the right amount! (Fluid level in the reservoir should be down ~1" from the neck once the engine is run & coolant circulates to eliminate any air bubbles from the cooling system; this space in the reservoir allows for coolant expansion.

Above: Cylinder head top radiator hose & thermostat housing opened up (4mm Allen Wrench), ready for parts. (The original O-ring sits in a recess in the upper thermostat housing flange. Don't loose the top O-ring.)

Above: New lower O-Ring and lower aluminum adapter set in place; silicone grease is recommended on the O rings.

Above: Upper aluminum thermostat adapter set in place

Above: When properly assembled, there is no gap where the upper housing meets the head.

With the thermostat installed, once the water temperature is up & the thermostat is working, the water temp reading is holding at ~156 degrees F at full throttle on around test on a ~62 degree morning.

NOTE: wile disassembling the engine in late August of 2019, I found that the frame of this thermostat had separated on one side where two brass tabs are inserted through slots in the end plate and bent inwards. I managed to squeeze it back into shape in a small bench vise, and then re-bent all four brass tabs more thoroughly to the inside to keep it together as intended- the photos below tell the story.

08-16 UPDATE: My THOR 250 setup uses a Powerfin two blade propeller set with a 57-3/4" diameter. This is a ground-adjustable pitch propeller and each blade is adjusted individually. Pitch is adjusted to limit the THOR 250 engine's top end RPM to a maximum of 7500 engine RPM; I'm presently adjusted for ~7350 RPM max in flight. (UPDATE: Replaced with a two blade 56" GSC Tech Series prop with the Urethane leading edge inserts in 2019)

When previously using the 56" diameter 3 blade GSC prop which Leon had supplied with this quad, I had to reduce the pitch on the three blade set to 9.1 degrees of pitch at the 75% of blade radius point, in order to get the engine top end RPM up to 7500. (I am using a digital angle gauge purchased from Harbor Freight for $30. Blade pitch is referenced to the center flat surface of the prop hub; that means that you set the gauge to zero against the face of the prop hub, then measure the angle of each blade at 25% in from the tip on the blade's back surface.) This pitch setting allows the THOR 250 to run at 7500 RPM at full throttle at 9940 ASL, but it was not really generating the thrust and speed that I need at this setting.

I've now switched to a two blade propeller setup using two of these blades in a different mid-part / hub; blade pitch setting will be steeper on the two blade setup. I'm now flying it at 12.5 degrees pitch, which does generate better thrust and airspeed. Presently for flying form ~10,000 feet ASL flying site: main jet is #118 (or #120 used later); Pilot Jet is #35 ; Air Screw is set to 7/8 to 1 turn out from bottom; Jet needle now has the clip in the second to the top slot of 5 for a leaner lower mid-range run. I'm running a 50:1 gas-oil mix, using 91 octane gasoline and 2 cycle synthetic mix oil.

TECH NOTE: GSC BLADE PITCH ADJUSTING: My technique for getting fine changes in blade pitch while making the adjustments: with all bolts loosened to no tension on the prop center hub and both pairs of prop blade mounting bolts, I'll simultaneously rock each blade forward and back within the prop hub while exerting the twisting force. A fine index mark on the base of each blade where it enters the prop hub helps me to see the slight changes in pitch angle while doing these adjustments. Once I have both blades at the identical angle, I snug down the hub bolts lightly, and push both blades fully FORWARD. The idea with this is to get the tracking of both prop blade tips as close to identical as possible (- within 1/16"). I'll do some final tweaking until they match as closely as possible. I use a measuring stick from the side cage frame tube to insure that both blade tips are tracking the same distance from that frame tube as I rotate the prop, recheck the pitch angle once more on all blades, and then tighten all bolts to the 100 foot-pound torque recommended by GSC.

Above: I added an enlarged metal instrument panel in order to mount the Fly Henry PPG Meter as well as the smaller water temperature gauge. ALL one might actually need beyond the water temperature gauge is an EGT gauge and a tachometer, but this PPG meter combines many additional capabilities. I also bought the fuel tank level probe so that I can have a fuel level display while on longer cross-country flights. [NOTE: this PPGmeter malfunctioned / died after 38 hours of operating time; it lost the internal temperature reference, went into alarm mode with constant flashing messages, and would not disable / reset as it should. So I removed it and installed a TinyTach for RPM readout, and installed a separate digital EGT meter. 

On the left, I mount my Garmin GPS; it has topo maps loaded as well as offering an ongoing readout of altitude and relative ground speed. The PPG meter offers a readout of EGT, CHT, Engine RPM, Fuel Level, current flight duration,and hour meter. It stores minimum and maximum readings for later display. The EGT readout is especially helpful in establishing the optimum jetting for full throttle running. [10-2017 UPDATE NOTE: I now use a newer Garmin GPSmap64s, mounted on with velcro on the gear pack.]

On the right is the ignition control box for this Dual Ignition Polini engine. The top toggle switch is the master on/off switch; the green LED at the top right glows when this switch is turned on. The bottom center switch is the electric start push button momentary switch. The two red push button switches are used to test the ignitions by disabling each of them temporarily and separately. The right is for the magneto ignition; this normally OPEN switch is pushed to short that ignition to engine ground, thereby killing that ignition spark. The left red push button switch is a normally CLOSED push button switch; when it is pushed, the secondary electronic ignition is opened, isolated from it's 12V power source, killing that ignition spark. The engine will run on either ignition separately. It runs very smoothly and reliably with both ignitions active- especially noticeable when doing test runs at the full 7500 RPM.

To kill the engine in a conventional aircraft installation, the top master toggle switch is toggled to the left, killing both ignitions. However, when flying a PPG, we typically have our hands full of the control line / brake toggles while landing, so reaching for the toggle switch without disturbing the canopy isn't reasonable- it's simply not safe. So there needs to be a way to kill the engine with a kill switch on the throttle handle while making a landing approach, or whenever the pilot wants to immediately shut down the engine.

Note: My implementation of this dual ignition kill function from the throttle grip is being done with a DPDT momentary push button switch & some extra wiring running into the throttle handle.

I accessed the engine control box's wiring harness connections for the magneto ignition [pale blue wire in the photo below= hot & black = ground], located back near the engine, to add in the wires for the kill switch connections (coming from the Normally Open contact section of the DPDT Momentary push button switch which I added into the throttle's handle.)

To do the kill switch wiring for the secondary electronic ignition, it's necessary to interrupt the 12 volt supply to that ignition. That 12 volt supply comes into the ignition wiring unit on the violet wire with the black stripe shown in this photo below. The heavier pair of Red & Black wires runs inside the sleeving over the throttle cable down to the second Normally Closed set of contacts of the momentary push button switch in the end of the throttle grip.

Below: The under-side wires use the COMMON and N.O. (Normally Open) contacts to ground the magneto ignition when the switch pushed. The top heavier wires use the COMMON and N.C. (Normally Closed) contacts to OPEN the 12 volt power to the electronic ignition when the switch is pushed.

Above & Below: The new dual ignition kill switch mounted in a plastic end cap, and the final finished throttle.

Above: This photo shows the place where I drilled a hole to install the EGT sensing probe for the Fly Henry PPG Meter. It requires dismounting the exhaust system from the engine to do this installation. From my perspective, having an accurate EGT display is the key to being able to optimize the carburetor jetting & adjustments- to KNOW that the engine is running at the optimum temperature across the full power range. That's my approach to knowing that I can rely on my engine at all times.

The CHT thermocouple that comes with the Fly Henry PPG Meter is the type that has a ring mount that goes in under the spark plug- it's easy to install.

I was a bit challenged with the sensor lead lengths being short from the PPG Meter on the control panel back to the Engine's location on this PPG Quad installation- the EGT probe wire just barely made it! Their web site says that the leads have since been extended another 6", which will be handy for trike / quad setups.

Above: HANG LOOPS - Position and lengths During my hang test to define where I needed the main hang loops to be connected to this PPG Quad's lower frame for proper balance with me as the pilot, I found that I needed to locate the loops as shown in the photo above. I also found that the length of the hang loops as provided by Skycruiser were long enough so that, when hooked in, the trimmers on the risers on my Mac Para Muse 3 XL wing would interfere with the hang guide loops on the top frame. I've had a trimmer latch hang up on my BLUEHAWK trike in the past, & with the combined weight of the heavy trike plus pilot, I could not get it free without landing. It was a go-around with a lot of opposite side brake held in to compensate in order to land & free up the trimmer.

Since then, I'm quite fond of rigging a low hang point setup where the wing's trimmers are clear BELOW the riser guide rings. That makes them more accessible for possible in-flight trimming adjustments, while also keeping them where they can not hang up on the upper frame guide rings.

Also shown in this photo are the extension loops I fabricated for extending the bridles from my APCO Mayday 18 Bi reserve parachute. An identical pair of 24" long extensions were sewn up from 18.5 kN (4047#) 1" tubular climbing webbing, with a center stitched section where three layers overlap for 4". (Stitching also extends beyond this center overlapped section.) I machine sew these with a heavy black bonded polyester thread that's specified for this application- it's really strong thread that's also very UV exposure stable.

Above: My reserve parachute is mounted to the lower left from the seat; the container is strapped to the PPG quad's frame in 5 places to keep it solidly & safely in position. The bridles are run up along the frame structure, held in place with 18# cable ties to keep the bridles & their extensions stowed out of the way. The near-side bridle's un-deployed extra length is overlapped & stowed along the upper frame tube, held in place with light cable ties. The bridles (with their necessary extensions) also run down through the riser guide rings on the upper frame members, and are always connected to the same caribiners where the wing riser connection loops also attach to the low hang loops. In this setup, if it's necessary to toss the reserve / rescue parachute, the same PPG Quad hang angle will be preserved, an the quad will come down on it's wheels- rear wheels touching down first.

PWK 28 CARB: Jetting, Adjustment, Tuning

Above: Keihin PWK 28 carburetor and the air box / silencer setup. The upper brass screw is the idle speed adjustment. The lower brass adjusting screw is the idle & low speed AIR MIX screw. Adjusting this screw out CCW increases the amount of air / leans the low speed mixture.

The main jet is accessible through the larger aluminum bottom cap. A 126 main jet was supplied in this carb when the engine came from Polini, which is appropriate for sea level & low altitude operation. In spring of 2018, when flying over Colorado's central Rockies and flying from an airfield at close to 10,000 feet ASL, I'm now using a #122 main jet.

[NOTE 06-2018: for flying at 1100 feet ASL in AZ earlier this year, I had to go up to a #132 main jet to get the EGT down into the optimum range.]

The prop you have and it's pitch determine how it loads this powerful motor. (With too much prop pitch, the engine can not run up to full RPM, regardless of the mixture.) The main idea is to prop a motor so that, at wide open throttle, the prop load limits the maximum RPM of the THOR 250 engine to 7500 RPM after the engine is broken in. More pitch can be used on an adjustable pitch prop at lower altitudes, where the engine can produce more power in the denser air.

UPDATED INFO: The stock pilot jet that came installed was a #45; the idle & low end mixture was richer than I wanted at my higher altitude. I next tried a #40 pilot jet, then a #35. I Tried a #32 for quite a while, but it acted lean on cold morning starts, so I have switched back to the #35. This should be ideal for running at 10,000 feet & above. I've also moved the jet needle's clip to the 2nd from the top of 5 slots, for a leaner run & quicker pickup coming up off idle. Air screw is set from 7/7 to 1 full turn out from bottom.

Above: the PPGMeter shows CHT on top left, EGT on lower left, flight / engine run on top right, and present RPM on the lower right.   Unfortunately, the PPGmeter malfunctioned after 38 hours of flight time and had to be removed.

TECH NOTE: The throttle cable must travel to move the slide up 28mm in order to open the slide completely on a 28mm throat carburetor - to give you maximum air/fuel flow for correct top end tuning and maximum power output from this motor. When I received my SKYMAX quad, the throttle cable setup was only opening the slide about 23 to 24 mm- not what was needed. The photo below shows the modification I made to the AVID throttle handle /housing / cable adjustment locking device to get maximum travel from fully closed to fully open.

Above: Throttle cable length rough adjustment is primarily done at the throttle lever. To be able to get this cable lock's allen set screw set where I wanted it, I drilled the added hole in the side of the throttle grip housing so that the allen wrench could be inserted through this hole to lock the cable at the desired length. The threaded adjuster on the carburetor's top housing has only limited adjustment range- not enough for what was needed here.

Above: A 4AH 12V AGM battery handles the starting job for this engine nicely. The hand pull rope flash starter is also included. The manual includes the instructions for it.

Above: A closer look at the lower end attachment of the new 8000# rated low hang loops in their position on the lower frame tubes.

Above is a view of the right side rigging showing the new 22.5" length low hang loops; this brings the risers down through the guide loops far enough for the metal trimmer buckles to be completely clear below the hang loops when the trimmers are set all the way out for more forward speed and wind penetration. Their lower ends are looped around the lower frame tube straddling the frame tube bolt just under the seat; this bolt will keep this loop from sliding forward or back on the frame to keep the hang angle consistent. You can also see my modified pack which fits within the forward lower frame in front of the seat.

Above: After only 5 months on the engine (while having the tarp cover over it to keep it out of the sun & weather) I was doing an inspection and noticed the start of these cracks in the rubber boot that connects between the carburetor inlet and the airbox / silencer. This photo clearly shows two lines of stress about 1/8" apart running around the circumference of the rubber boot. When I removed the airbox & it's connecting boot from the carburetor, I discovered that the carburetor inlet has a short "velocity stack" mounted to the outside of the inlet throat. And further, I discovered that this velocity stack had a raised rib around it's outer circumference- just exactly where the rubber connecting boot developed the stress cracks. Upon examining the interior of the mating rubber boot surface, however, I found that there was NO RECESS molded into the rubber boot to match the rib on the carburetor's velocity stack... No wonder the rubber was showing stress crack failure so soon into it's service life!

Above are the results of my work to remedy this issue. I first removed the 'velocity stack' ring from the carburetor, then proceeded to remove the offending raised ridge around it's outer surface. I used a Dremel belt and disk sander unit to do this, ending up with a smooth outer surface which matches the interior of the rubber boot.

Finding a listing for a replacement rubber part online for this Polini engine was elusive; while getting a new rubber boot would take time, I decided to do a reinforcing repair to the original one. (This is my already proven technique; I've used it before on similar parts on my Hirth F33 engine, which have since been in service for a long time.)

I had been told in the past by the owner of an auto parts store that the Permatex BLACK silicone adhesive / sealant was the strongest and most oil and fuel-resistant product to use on a molded rubber part like this. It's shown in it's package. The other tool you see in the photo above is a fly tier's bobbin loaded with a spool of kevlar fly tying thread. This is incredibly strong, tough thread; if you tried to break it by hand bare-handed, you would cut into your hands before ever breaking this thread- it's that tough. (You can find it on Ebay in an assortment of colors, or buy it at a store that sells fly tying materials & tools. The bobbin makes handling the thread far easier than it would be to work with the spool of thread alone.

The first step is to remove all traces of the 2 stroke mix oil from all surfaces and any cracks in the rubber connecting boot. For this job, I use an automotive spray 'Brake and Electrical Parts Cleaner'. All traces of oil must be eliminated so that the black silicone adhesive can bond strongly to the rubber. Once it's clean and dry, the first application of the adhesive is worked thoroughly into any checks and cracks to fill them completely, and then a coating is applied to the outer surface in the stressed area.

Next, while the adhesive is freshly applied and wet, the kevlar thread is wrapped around the entire stressed rubber area, criss-crossing back and forth to create a weave that crosses all of the stressed area and adding the desired strength. This is the area where the clamp will be reinstalled later. Once the thread has been wrapped on adequately, more of the Permatex black silicone is applied over it to end up with the thread being completely wetted and covered with a thin smooth layer of the adhesive. Make sure the inner surface is smooth- wipe away excess, then set the part aside in a warm place to let the silicone cure out for about 24 hours.

The resulting reinforcing repair is what you see in the photo above; all cracks filled & bonded with a layer of kevlar thread which results in the part being stronger than the original un-treated rubber part. It's ready for re-assembly in one day.

Above: I also noted fine cracks beginning to form in the engine-to-carburetor rubber boot - likely from the weight of the carburetor and airbox / silencer continuously pulling down on this rubber part. Again, this was seen within about 5 months of the factory assembly of this engine. This was something that I wanted to modify.

[You can see in the photo that I had installed a rubber tube stand-off and cable tie support from the upper front edge of the air box to the PPG frame to minimize movement & secure the assembly forward well clear of the prop, but more modification in the carb / airbox mounting was needed.]

My approach was to remove this rubber part, thoroughly clean it, and add the Permatex black adhesive sealant and the reinforcing thread wrap to the outside where the stress affects were found.

Taking the weight off this rubber mount part was my next project. I decided to simply add the stainless steel wire support link shown in this photo between the carburetor's top cover mounting screw and one of the mounting screws form the electronic ignition module's mounting plate. I used two electrical ring connectors on each end of the wire to provide the mounting rings which the screws pass through, then adjusted the length so that the rubber carb mounting boot was no longer being asked to carry the weight of the carburetor and air box / silencer. The result, as seen in the photo above, is effective- the SS wire is adequate to handle the task with a minimum of added weight, and easily removable / replaceable for normal carb servicing.

You'll also note in the photo that the throttle cable boot at the top of the PWK 28 carburetor was already cracking and failing after only ~5 months; I've temporarily wrapped it with some tape, but plan to do the kevlar thread & silicone sealant treatment to this part to when I next pull the carburetor top cover.

So Polini may need to improve the quality of their rubber carburetor mounting parts. Maybe they are already in the process of doing so- I hope so. But now I know to keep watching these areas during my pre-flight inspections.

Above: UPDATE: This is a rather disturbing discovery. The motor mounts which came with my engine are failing after only 15 months in service (and relatively limited flying time.) I have now replaced these with a different through-bolt bushing setup ! With the stock upper motor mounts, there is nothing to stop the engine from leaning way out of position if these upper motor mounts fail!

These are the parts of the replacement through-bolt motor mount setup.

New through-bolt upper mounts installed.

UPDATE: I later went back to using a new set of the stock vibration damping upper motor mounts; inspect regularly for signs of cracks, as they are under tension, not compression!

Here's a pilot's-eye view looking up at the Mac Para Muse 3 wing in flight in Colorado's incredibly blue sky.

[NOTE: In September of 2016 I ordered the new MacPara Charger reflex airfoil wing in the 31 square meter size. At my wing loading in this10,000' ASL thinner air, my combined rig cruises at 37+ MPH in full reflex trim- great for the long cross country flights I enjoy. The wing tip steering is also a sheer pleasure to use; only slight deflection with minimum pressure is needed for minor course corrections. The wing responds well to deeper deflections of the wing tips with modest pressure on these wing tip control lines.]


My Strobe Light can be quickly mounted just above the radiator, using nylon tie wraps. It is now wired wired through an on/off switch into the 12V system. Operation with a strobe visible from 3 miles allows extended flight operations 1/2 hour before sunrise and 1/2 hour after sunset. It also makes this PPG more visible to low flying aircraft.

On two occasions my left rear wheel has bound up on the axle while doing ground run tests- it stopped this Quad from rolling rudely and abruptly. This would have been bad news if I had the wing in the air at the time. I first removed some excess powder coat paint from all four axles after finding that the inner sleeve of the bearing was not sliding over it, but that did not cure the problem. The wheel bearings needed to be shimmed away from the inner stop plate.

So I went to my local True Value hardware store and bought 8 "Machine Bushings", 5/8" inside diameter in 18 gauge thickness. I place one on the inner & one on the outer side of each wheel. I use a large 5/8" washer on the outside of all of this as an outer wheel 'safety stop flange' in the unlikely event of a wheel bearing failure. The new NyLock wheel nuts are set in close with just a slight bit of clearance, where the outer safety keeper washer can still be freely turned with my fingers. All wheels are turning freely & smoothly now, & should not be any cause for further concern.

After ~16 months, the lightweight Lawnpro 13" tires were all developing checks and slow air leaks. A temporary remedy is to add tire sealant to these tires. It's working for now- they are all holding pressure.

I have now installed a full set of replacement 15x6.50-6 smooth ribbed tires (similar to the tire shown below). These are a tougher 4-ply garden tire, weighing only 5.5 pounds each. They should be more 'cactus-resistant' than the light 13" Lawnpro tires.

Above: Waterproof Fabric Cover; designed for a boat up to 16' long with a wide beam. This fabric cover is wide enough to cover this PPG Quad on the trailer nicely, including covering the rescue parachute while it is mounted on the quad. 1 long Nylon strap with 5 quick-release buckles keeps it in place.

Above is one of the cage edge line support fixtures I made from tough and flexible plastic.

Above and Below: I've mounted a 58" Powerfin Propeller for testing. This is the earlier style hub

I set the initial pitch at 12.5 degrees (measured at 75% of prop radius out from the center.) A test run here at 10,290 feet ASL in 59 degree dry air gives a peak RPM of 7400 to 7420 RPM- very close to the ideal of 7500 RPM for max H.P. output.

This propeller is running smoothly and providing good thrust; it's a good match for the THOR 250 engine flying at this altitude.

[It was later replaced with the 56" two blade GAS propeller.]



Adding An Extended Prop & Line Guard Hoop with 7" Stand-Offs To The Prop Guard Cage


Above: since the THOR 250 engine has the centrifugal clutch on the prop drive, when the engine is shut down the propeller has a lot of inertia and easily continues to windmill. During a landing, when laying the wing down after roll out & stopping the quad, the lines could easily get wrapped around the engine's output shaft- the prop cage did not extend far enough to protect the long prop blades and keep the lines from dropping between. The solution: Add an extension to the prop guard cage. Net added weight : only 3.1 pounds!

Above: after shaping the hoop using a 1/2" EMT conduit bender and my homemade Hip Bender the six extra stand-offs were fabricated and welded to the extension hoop. This is the result after painting, ready for installation.

The extension guard hoop is mounted in place to the original aluminum cage with 8 stainless steel worm drive clamps. This photo shows that the 58" propeller is now protected within the extended cage. It has good clearance.


Prop blade end clearance is over 1.6" minimum.

A rock kicked up by a wheel on the quad caused this damage to one GSC maple blade's tip. I filled it with slow setting epoxy, shaped it carefully, and re-varnished the blade before adding the stainless steel tape to the leading edges of all blades.& I have repaired other blade rock damage since this; when flying from the irregular ground of South Park's short grass prairie, this is an ongoing hazard. I have put in a lot of time smoothing ground and picking up loose surface rocks to minimize the possibility of a rock getting kicked up into the prop, but it does happen. Johnny Fetz showed me some good techniques for prop repair; that info has come in handy on several occasions.

 I decided to remove the pull starter; I never use it, so that's 1.1 pounds that I don't need to carry. The internal pawls degrade from vibration during normal flight, and require ongoing replacement to continue to function properly.& The electric starter works well, although I'v had to do maintenance work on it too periodically;& but this engine starts very easily with it.


After completing these modifications, I have flown many flights over South Park, starting from& one of two launch points: one at ~9680' MSL and another& at 9944 feet MSL, and eventually climbing up to a maximum of & ~15,029 feet ASL according to my Garmin GPS. Photos from this and subsequent flights are now being combined on the PHOTO GALLERY page- CLICK HERE to jump to that page.



 Flying longer cross-country flights with a PPG leaves a pilot gripping the hand throttle fairly tightly for extended periods of time- often with your hands held up in a high position while also holding the control line handles. For me (& a lot of others I suspect) this can lead to tired / aching hands, reduced circulation, and colder hands during cool weather / extreme altitude flying.

I can adjust the trimmers on my MACPARA Muse3 wing for close to hands-off flying as far as the control lines are concerned; when let go, they're easily in reach right against the pulleys, but when I'm cruising cross-country, I can have my hands off the control lines for extended periods of time. This allows me to work with my cameras and GPS controls while flying. But up to now, that throttle control has always been tightly in my grip... so I decided to design a simple device to use as a 'Cruise Control' - to keep the throttle cable pulled in the desired amount for extended periods of time, whether at full-power climb when heading for the higher altitudes, or at an intermediate setting to maintain a given altitude in 'cruise' mode. The photos below show what I came up with.

The device is a type of variable-effect  stepped shim which, in use, is slid into the throttle housing straddling the cable to keep the throttle lever from being able to be pulled back up by the return spring within the carburetor. The material I had on hand was a piece of Basswood 1/4" x 1/2" x ~1.5" long. I cut a slot up the center from one end for about 3/4"; the slot is wide enough to straddle the throttle cable without hanging up on it. A hole was drilled in the opposite end where a ~6" long section of spectra 200# fishing line is tied; the other end is tied to the heavy wiring running from my throttle's kill switch. This line simply keeps the cruise control device from being dropped / lost.

When the device is slid all the way into the throttle housing, the throttle lever is held at close to full throttle position.

When the device is slid part way back out, a 'level cruise' RPM / throttle opening can be selected with a simple bit of trial and error of sliding the shim device either in or out.

When not in use, the device slips snugly inside the throttle grip, available for it's next use.

Being Prepared For An Engine Outage Over Heavily Timbered Terrain

When planning cross-country flights over the terrain we have here in the central Colorado Rockies, It's always good to give some thought to how to deal with unanticipated situations. One of those unlikely but possible situations which a PPG pilot might someday have to deal with is that of having an engine or wing malfunction while flying over timbered terrain.

As pilots, we do our ongoing maintenance and pre-flight checks to assure ourselves that our equipment is in serviceable condition, ready to perform reliably and continuously. But it also doesn't hurt to be prepared for a possible landing in less than favorable terrain. Being hung in a tree in your gear is a remote possibility; being able to get yourself down safely if you ever are is close to priceless!

Above is a "Self-Belaying Kit" which I will carry when on longer flights over the heavily timbered terrain. It consists of :

[1] A light weight climber's seat harness and Chest harness and 1 locking carribiener; I built this one from 4000# + 1" tubular climber's webbing and the heavy bonded polyester thread which I use to sew the low hang loops for my Quad. (The adjustable waist belt with the cam buckle to which the main leg loops are sewn just helps keep the leg slings in place; the main leg sling loops carry all of the load when in use. )

[2] A "Figure Eight" descending / rappelling device

[3] A 100 foot long length of 6mm quality climber's accessory cord

[4] A good pair of gloves for rope handling with leather palms

[5] A 4 foot section of 1" climber's webbing (Or a good quality Cam Buckle strap) & another carribeiner for use in fashioning an anchor point.

[6] 30' of strong 4mm utility cord

To use this gear if hung in a tree, etc., a stranded pilot (transformed then into being a climber) would first carefully get out a shorter section of rope and tie them self and their gear off to the tree- no need for a sudden unscheduled early decent! Next is to put on the climbing seat harness and rig the carribieners and figure eight as shown; the loose end of the rope in the above photo would be the one tied to or looped around a solid branch or part of the tree trunk, or tied to the webbing anchor point. Using the rope run through the figure eight more than once increases friction, and running this 6mm rope through doubled is fairly standard self-belay procedure; the center of the rope is looped over an anchor point attached to the tree so that, once on the ground, the rope can be retrieved from the anchor point for further use. [Mammut's 6mm cord has a breaking strength rateing of 1700 pounds. Sterling Rope's 6mm cord is rated at 1978#]

[A book or website on climbing ropes, rigging, and use and a bit of hands-on practice should further help in an understanding procedures for rappelling using a descender. Professional ski patrol personnel carry a similar kit for self-evacuation from a chair lift if the lift is shut down while they are on it; fire fighters carry similar gear with high temperature-resistant rope. A practice self-belay decent from a very modest height might not be a bad idea for those unfamiliar with rope & self-belay techniques. A Gym with a climbing wall and instructor might be a good place to familiarize yourself with the equipment and techniques; that info is beyond the scope of this web page.]

Above: REI gear. (I changed out to the buckle for my PG kiting use.) The figure eight chest harness helps keep you upright while doing a descent if your technique is less than perfect, and you may like the solider feel of rigging that way. REI sells both- I bought the set from them many years ago. I have used this climber's seat harness a lot for kiting my PG wing. The gear I'm carrying is lighter and more compact.

(Above & Below): My self rescue / rappelling kit as shown in the previous photo weighs 46 ounces, packed in this light weight mesh bag, ready for use. I have a converted day pack which is mounted to the forward lower frame of my Quad, where I carry this gear in easy reach. I can carry this kit, a GMRS radio, my cell phone, and a survival kit in the mounted pack and still have room for my wing's empty carrying bag and other items.

(Above): This is a photo of most of the main items included in my survival kit (28.75 ounces) which includes 2 Orion "Skyblaster II Alert signal flare launchers, a lightweight emergency bivy sack (3.4 ounces) and two mylar space blankets, two 30' lengths of 200# Spectra line, a LOUD signaling whistle, plus a small first aid kit including a roll of tough nylon tape, a chemical hand warmer, a fire starting kit, a mini 47 lumen LED flashlight & extra AAA battery, 3 protein bars, a light warm poly stocking cap, paper towels & TP, and a few other useful items. (An 8 oz. water bottle is not shown in this photo, but can be carried in the pack.)

I fly with the reserve parachute mounted and with these emergency survival and self-rescue kits always along because I'm inclined to fly over high rugged remote areas most days - that's where I live & fly- and I figure it's worth having along this 4.5 pounds of emergency gear. That's my personal approach - a part of prudent PPG flying in this beautiful and rugged part of Colorado's high country!

I carry a Midland GTX1000 GMRS radio which has up to 36 miles of line-of-sight range, while leaving a matching radio with someone back on the ground, and I leave my flight plan with them. If I do have to put down somewhere other than at my launch point, I'm fairly well prepared.

For photos taken during my flights, please CLICK HERE to visit the PHOTO GALLERY.

2017 Updates and Modifications

This update section was last updated on October 27th, 2017

Late in 2016, I had Andy McAvin ship me a new MacPara Charger 31 wing. This is a Reflex wing with a higher cruising speed when the trimmers are let out into the full reflex mode. In the thinner air here at altitude, with the ~380# total ready to fly wing  loading of the quad and me, I'm cruising at an average speed of 37 MPH. With trimmers pulled in fully for better climb angle, the speed is at 33 MPH. [In comparison, the 31m Muse 3 would cruise at 32 MPH when the trimmers were let out fully.]

 Really like the way that this wing launches and flies, and I find the wing tip steering very nice to work with. However, for use on my quad with it's riser guide rings on the upper frame, I had to do some modifications.

With the long travel trimmer adjustment range and the added wing tip steering, the Charger's risers are more complex and far bulkier than those on a simpler wing like the Muse 3 which I had previously been flying. The 2" I.D. riser guide rings provided with the Skymax quad (mounted on the quad's upper frame members to control the hang attitude)& were too tight a fit- the wing tip steering lines were regularly fouled / hung up and unusable, and other parts of the stock risers would also hang up in the guide rings, leading to problems and aborted takeoffs.

First, I fabricated a new set of low hang point loops that positioned the trimmer clamps where they were workable below the riser guide rings. Next, I fabricated a new set of larger 2-9/16" I.D. riser guide rings which offered more adequate clearance for the Charger's risers. I use the 2" guide rings to route the reserve parachute's risers separately, inside the upper frame members, and then down to the main carabiners on the low hang loops, keeping them clear of the wing's risers.


These modifications helped, but all of the extra D-rings and shock cord form the original rigging and routing of the Charger's wing tip steering lines running up through the middle of the riser bundle through the riser guide ring still left the wing tip steering toggles & lines fouled and unusable a large percentage of the time... so I decided next to try to remedy this aggravation.(This situation might not necessarily be an issue for a trike or quad rigged with a high hook-in setup, or for foot launch pilots.)

(The photo below shows the stock Charger risers through the 2 inch I.D. riser guide ring, with the stock wing tip steering line run up through the midst of the bundle.)

The photo below shows the original stock wing tip steering line rigging run through 6 D-rings with a lot of ~1/8" shock cord run back and forth everywhere.... when these risers are used through riser guide rings on a quad or trike setup using low hang point rigging, all the extra D-rings & shock cord result in wing tip steering line fouling and hangups far too often.

My solution: The risers come with two brake line pulleys sewn in place on the risers. On my quad with it's low hang point rigging, I need to use the upper pulley for the brake line. I use the upper brake toggle retaining magnet (the right one in the photo below) to park the brake / control line toggle while in flight. ( I use the wing tip steering all of the time while cruising; I only use the brake control lines during takeoffs and landings.)

This leaves the lower brake line pulleys unused, and so available to be re-positioned and re-purposed in a cleaner wing tip steering line setup. The idea is to end up with the wing tip steering line routed to bypass the riser guide ring completely, eliminating any of the fouling and hangups.

The photo above shows the 2nd lower brake pulley sewn in it's original position, down low and close to the trimmer clamp. I carefully pulled the stitching to remove it with it's webbing loop. Next, as shown in the photo below, I sewed it to the existing upper brake pulley's webbing loop with heavy duty bonded polyester thread; it's solidly in place. I then proceeded to remove all of the 6 un-needed D-rings and shock cord, and the old low eyelet through which the wing tip steering line had once been routed leaving nothing left for lines to hang up on. (I later found that the extra low position brake toggle parking magnet was hanging up in the riser guide ring, so I pulled it's stitching and removed it, too.)

The important thing to keep in mind if someone else decides to clean up their wing tip steering line routing for a quad /trike with riser guide rings and low hang point rigging is to only cut the thread carefully- do NOT damage the risers themselves! A seamstress's  'stitch puller' tool is very handy for this type of careful work.)

The photo below shows the finished result- the wing tip steering line toggle still parked on it's  magnet and snap fastener, the line run freely on the outside of the riser bundle up through it's own pulley. It performs flawlessly in flight now.

While flying my early flights with the Charger wing, I found that the torque from the Polini THOR 250's gear drive resulted in a left turn tendency when both trimmers were set to neutral. Since I wanted to be able to fly the full trimmer range during normal flying, I was looking for a way to compensate for this left turn tendency... after all, for long cross country cruising flights, it's really pleasant to have the rig set up for hands-off straight line flight for extended periods of time.

I noted in watching the wing that it only takes about 3" of inward deflection on the wing tip via a modest pressure on the wing tip steering line to compensate for that left turn tendency. On the Charger Riser control line setup, there is a short line which I have come to refer to this 495mm long " stC1 line" as a 'snubber' line connected between the C riser quick link / mallion and the wing tip steering line; by shortening it the right amount, it can also function as a 'torque compensator' device.

I first made up an adjustable parallel line with a 3/4" wide strap and cam buckle- similar to the regular trimmer cam buckle and strap setup. Once in flight, I pulled in the adjustable line until I had a hands-off straight line cruise. After the flight I determined that I needed a 'stC1' line that was 100mm shorter to keep that right wing tip pulled in the required ~3" to result in a straight ahead hands-off cruise.

So I made up a 395mm long line from 220 pound test Spectra line and installed it in place of the original 495mm green line. It's been performing as intended for many long cross country flights since then. You can see the short grey line in the photo below.



[Updated 06-20-2018] I like the electric starter on my POLINI THOR 250 engine... but it does have it's own maintenance requirements. James Weibe, in his BELITE blog, wrote of having to repair a broken internal brush wire; nice to know.& I have had to do the same myself.. I ended up having to solder both brush wires to their terminals; it seems that the spot welding of the fine strand copper braid to the terminals results in a heat-induced fatigue of the copper. Since I did the initial soldering of the brush wires, I'v had to go back in and re-solder them on several occasions over the course of the first 80 hours of flying time. I've emailed Polini directly about this issue, requesting to obtain a replacement set of brushes, brush holders, and the attached pass-through terminal. This is an ongoing maintenance requirement on the THOR 250's electric starter which can be done if the parts are made available. I'll add info to let you know how Polini responds on this issue.

The second starter issue deals with the starter's output shaft... the splined section is a separate piece of metal which is inserted into the starter armature's shaft, and crimped in place. It can work loose in service.... not an ideal design! The photo below shows the splined short shaft sitting inside the engine housing, visible when the starter was removed. The only good news is that it can't escape this area to get deeper inside the engine to cause more extensive damage. The bad news is that once it happens, you get to fix it.

 The photo below shows the recess in the end of the starter's armature shaft, and the insert shaft's smooth end laying along side just below to the right.

My solution was to first thoroughly clean both parts with spray 'Brake and Electrical & Parts Cleaner", to remove any oil , etc. I then put some thin CyA glue (super glue) into the armature shaft recess, inserted the smooth end of the splined shaft, and seated it to bottom solidly. Once the glue cured well, the starter has been back in service reliably for many long flights. The photo below shows the reassembled shaft. [06-2018 Update: Shaft staying in place since being glued last year.)

The photo below also shows the brass pass-through terminal for the starter's red positive wire from the starter solenoid; I have had two of these terminals break off at the sharp bend where it comes out the top of the pass-through plastic insulator. To repair this failure, I cut away about .1" of plastic so that a good quality wire could be soldered directly to the remaining brass terminal.

UPDATE JULY 21, 2018:: The starter motor's brushes have developed breaks in the brush wires several times during the first 100 hours of flying. I've solder them to get them back in service several times. (James Weibe first wrote of this issue in his BELITE blog several years ago onw; it seems to be a very common failure mode for this electric starter. Recently, when a brush wire broke away at the top of one of the brushes, I could not solder it to repair it. My email to Polini requesting either a set of replacement brushes, or alternatively information as to a source for suitable replacement brushes remained unanswered for over a month. Leon to also had nothing to offer, saying that I would have to source them myself.

After extensive searching across the web, I eventually sent an inquiry to; they replied promptly, asking for photos in addition to the dimensions (= 6mm wide, 9mm high, ~12mm long with the wire coming out of the top 6mm wide surface near the outer end) which I had sent. After sending photos, I again received a quick response, saying that their part # 70-109 might work. (19.95 list for a pair.) I found an Ebay vendor offering Rick's 70-109 with free shipping... they arrived this morning. I checked and found the brush size very close to ideal- correct width & length with terminals. The brush height was 9.5mm instead of the stock 9mm height. A bit of careful sanding on some 220 grit silicone carbide sandpaper resulted in an ideal fit. (These brushes sand easily- use light pressure, keep it even, and check for fit in the brush holder often.

These brushes are used in the starter motor for a Yamaha 225 Moto 4, late 1980's vintage; they are now primarily available from RICK'S, & distributed through many dealers across the country, and offered by several merchants on Ebay.

A replacement for the case pass-through terminal can be scavenged from an inexpensive starter motor for the Chinese made GY6 scooter; these starters are sold on Ebay for ~$16.00 with free shipping. While the starter housing looks similar, the 9 spline output shaft is a larger diameter- not a match for the THOR250's needs. The internal brush assembly is also different... but the output terminal is an exact match to the THOR250's terminal, and screws in place. You'll just need to solder one brush wire end / terminal to the inside end of the pass-through terminal.

With the Polini -sourced replacement starter motor costing over $200 & taking quite a while to get delivered, if you have the skills, maintaining your original electric starter is a reasonable option; with parts on hand, you can have the starter back in service very quickly- less than an hour in my experience.

The Polini manual starters are one of the more aggravating features of an otherwise fine performing engine; they really need to improve the design. It's made partially of plastic parts which self-destruct from vibration while the engine is running normally. While the 'pawl set' is relatively inexpensive, it's generally a too-frequent maintenance requirement if you are using only the manual starter. I had started my engine with the pull starter only 2 or 3 times, and left it mounted as a 'backup'.... but when I tested it, it would not engage. So I removed all of the pull starter components, and then glued a protective cover over the magneto flywheel opening where the manual starter had been mounted. This keeps debris from being able to get into the engine's magneto area.

UPDATE: There is a direct replacement starter, designed for the APRILLA RS125 scooter with it's Rotax engine, which mounts directly to the THOR engine case at the two threaded mount holes with the same bolts which were used to mount the Polini starter. The splined shaft matches when installed, engaging the starter spur gear to the same degree as the polini. It's a cleaner mount, not using the polini's bolt-on mounting brackets. The cost on Ebay as of July 2021 is around $30, including shipping from outside the U.S.

I prefer to solder on a wire and do some form of anti-vibration strain relief; I installed one of these to try in the summer of 2020. I had one stop working after about ~45 hours of flying time... but that's about the same life expectancy of the starter brush braided wires inside the stock Polini starter... The failure was due to an open in the armature windings that is un-repairable - the same fault which also killed one of my Polini starters. At roughly 1/6 the cost or less, having a spare starter on hand is very reasonable; the swap-out can be completed in about ten minutes.


The above photo shows the interior of the starter end housing which holds the brushes; the interior of the Polini stock starter is similar, dealing with carbon brushes which are held against the armature's commutator by springs which are trapped in the brush holders beneath the brushes. The problem with both the Polini stock starter and the APRILLA 125 replacement starter shown in the photo above is that the brush wires are spot-welded to the terminal. This spot welding weakens the brush wire right at the spot weld; after about 40 hours of flying time, the internal vibration causes the wire to break at that spot. Again, this hap[pens in BOTH of these starters.

The FIX: open up the starter and solder the end of the brush wire back to the terminal. This requires a ~40 watt soldering iron, solder, and a bit of liquid soldering flux. My best technique is to apply flux to the ed of the brush wire, then flow some solder onto that wire's end. I then add a bit of flux, and then flow a small puddle of solder ont the terminal just above where the wire had beel spot welded. I then complete the solder joint by re-heating the two solder areas together - it takes a very brief application of heat, and results in a reliable solder joint. (Don't let the wire move while the solder is re-solidifying.)

THE TRICKY PART: Getting the springs into the brush holders, sliding in the brushes over them in their positions, and then having them STAY IN PLACE while you get the end housing over the end of the armature. I know of no other way than to tie back each brush's wire to the outside with strong thread while the spring and brush are held in place with a finger. This is at least a three-handed job, so having a friend do the ttieing of the thread is very helpful.  In the photo below, you can see that the top brush 's wire is tied back to the outside with thread already; the lower brush is being held back in place, ready for the thread to be tied tightly.

In the photo below, you can see that added pieces of scrap have been inserted to further tension these threads, to givre full clearance for the armature's commutator to slide into the housing without hanging up against the brushes.

You will want to have the arrmatureout of the can housing with it's magnet before trying to reassemble the starter. It's a good idea to lightly burnish the commutator's copper segments before reassembly; remove any grit. I also add a slight bit of grease to the bearing surfaces of the armature shaft before reassembly. The photo below shows the armature slid in past the brushes, fully forward in the end housing; once in place, the threads are cut and removed.

The last trick is to hold firmly to the splined shaft's splines as you slide the rear can housing with it's magnets over the armature; you don't want the magnets to pull the armature out of it's place in the end housing . Then install the screws that hold the starter together.


 Below: The battery rides in a nice Velcro mounted nylon carrier, isolated from any frame vibration. On my Skymax, the wires would vibrate enough to eventually loosen the screw on one of the battery terminals... not so good a thing to have happen. So I added long cable ties around the battery and it's wires close to each terminal, so that the wires can not move. (I like to use lots of cable ties!)





In this ~70% density air, with my loading on this 31 square meter wing,  the GARMIN GPS's recorded track data shows that I need to get rolling up to 28 to 29 MPH to lift off and fly. landing approaches are also typically at about 28 MPH, until I begin to flare for landing, when the glide speed reduces below that approach speed. This 29 MPH required speed for liftoff results in a need for a fair amount of usable ground to get up to liftoff speed- especially when taking off from soft ground.

A 380 pound rig rolling across natural short grass prairie at 28+ MPH puts a lot of stress on the quad's structural members.

So the fiberglass axles and their steel mounting tubes on the quad's frame undergo a lot of stress when being used in these conditions, which necessitates some ongoing monitoring / inspection and maintenance. Even trailering the Skymax quad on poorly paved and gravel roads takes it's own toll, stressing fiberglass axles and the 1" ID tubes which carry these axles. Below I offer information on the items which I've had to rebuild, repair, weld, and reinforce on my quad due to these structural stresses.


Upon returning home from the Lost Stirrup Fly-In in mid July this year, I found that welds had failed on both of the main upright frame member mounting points, as shown in the photos below. This attachment is formed by welding a larger I.D. sleeve tube to the underside of the main lower frame's lengthwise tubes on each side to support the entire upper frame members, engine installation, and prop guard cage. A major amount of weight is carried by these two MIG welds  which are holding everything above up in place... The load is from above, but these welds are trying to hold it all up by being welded to the underside of the lower main frame. These welds both failed after less than 50 hours of flying time, and some trailering to and from flying sites within a few miles of home.

An added consideration is that the location of these welds is not easily visible during a pre-flight or post-flight visual inspection; I was unaware of the developing problem until they broke loose completely. The resulting downward movement of the entire upper structure also caused damage to the prop guard cage structure; luckily the prop was not damaged.

 Instead of tearing the entire rig totally apart and packing in in a box to ship back to Skycruiser, I decided to repair and reinforce these weld failures myself. Yes, it required totally removing the engine and control system so that I could work on it properly; but I had the equipment here to do it effectively and quickly.

My approach was to first trim down the remaining excess lengths of the inner lower ends of the upright frame tube members. Relocate them to a position ON TOP of the lower lengthwise frame tube members required removing the motor mount cross members. Once re-positioned, I added sections of steel angle iron to the underside of the damaged lower frame tubes bridging over where the metal had broken away.

The next step (once the pieces were clamped in their new positions) was to drill and bolt through the new attachment points with high strength 3/8" bolts and nylon insert lock nuts. The result is shown in the photo below. The resulting structure is very solid, and the downward-bearing loads are now carried on top of the reinforced lower lengthwise frame members. I feel a lot better about flying this quad after this modification / repair. It's now also easy to check these through-bolt attachments during my pre-flight and post-flight  inspections.


 I noticed damage to the rear fiberglass axles at the point where they enter the rear axle carrier tube; the fiberglass was showing fiber breaks at the point where the end of the steel tube made contact with the fiberglass on the top surface. The original retainer pin hole had been drilled vertically with only 3/8" separation between the pin hole through the fiberglass axle and the contact point with the end of the tube where I could see the fiberglass was failing.

On closer inspection, I also discovered that both ends of the axle carrying tube had developed vibration fatigue cracks on the top surface along the edge of the mig weld which connects the axle carrier tube to the quad's framework above. The next three photos below show what to watch for during your inspections.

The position of these cracks indicate that they may be the result of vibrational down-load stress forces during flight; the axles with their wheels on the outer end must vibrate constantly during flight. Arc welding does induce stress into metals do to the heating being concentrated in a very small area; that's why steel tube built light aircraft frames are built using gas welding techniques, and heat stress relieving is done on welded areas during the build after each weld is completed. Arc welded joints need ongoing inspection and monitoring for the possible development of these stress cracks.

After doing an overlay weld with my wire feed welder  as shown in the photo above, I smoothed down the welds and repainted the area. I drilled new horizontally oriented holes for the axle locating bolts well away from the fiberglass axle load stress point at the end of the axle tube. I then chamfered the inner ends of the steel tube to minimize fiberglass surface fiber stress in those areas before installing new black 1" OD fiberglass axles.

NOTE: I am now buying black 1" diameter round solid fiberglass sections for replacement axles from Max Gain Systems

(The black color may be more resistant to long term U.V. damage of the outer surface resins than light colored axles. I've had fibers raising from the surface of the original off-white axles in the past. I like the looks of the black axles.)



Below: While inspecting the quad, I noticed some deformation on the right end of the front axle carrier tube where it was welded to the upright piece. (This is rather rough ground I fly from, and the takeoff runs are much longer than when flying in denser air at lower elevations.  I'm likely close to 380 pounds ready-to-fly weight when I'm rolling at 28+ MPH across the terrain to get up to flying speed. So for my purposes, I want tough axles under my rig. Being light weight is a lesser consideration, versus a need for structural strength and longer-term durability. )

After straightening, I welded on a close to full length piece of reinforcing steel angle iron (3/4" x 3/4" x 1/8" thick.) Added weight is modest, but the added strength is very welcome! I expect long service after this modification.

In July of 2021, I cut away the 1" ID axle carrier metal tubes which had been repair-welded on several occasions, and replaced the old axle setup with 1-1/4" fiberglass axles carried in 1-3/8" O.D. Chome Moly Steel tubing. Old welds were cut away with a rotary cutting disc tool, and the new tubes MIG welded in place. Below is a photo of the old front axle assembly; below that are photos of the new larger axle carrier tubes being welded in place.

BELOW: The new front axle is a continuous 34" long section of 1-1/4" solid fiberglass; it is kept centered with a single 1/4" stainless steel bolt. Plastic flange bushings on the front assembly pivot bolt were replaced with brass flange bushings, as the original plastic bushings were disintegrating. The new 1-1/4" axle installation is the same weight (within one ounce) as the reinforced 1" front axle assembly which it replaced.

BELOW is the new 1-1/4" ID rear axle carrier tube being welded in place after the original 1" ID carrier tube's welds had been cut and ground away. Further welding was completed after this photo was taken. The new rear 1-1/4" OD solid fiberglass rear axles extend about 6" into each end; they can optionally be slid in another 6". Optionally, a single piece full length rear fiberglass axle could be slid through this Chrome Moly steel tube. Holes for bolting the axles into place are located well away from the ends of the steel tubes to minimize stress in the fiberglass axles where the holes are drilled; these drill hole ares are the places where splits and damage has occurred in the past on the 1" fiberglass axles.

NOTE: A broken right front 1" axle occurred when hitting a hole dug by a badger during a high speed takeoff run; the result? The axle carrier steel tube dug into the ground, flipping the quad on it's side, with the earlier lighter build prop guard cage collapsing into the prop (which was turning at full RPM at that moment) destroying both the prop guard cage and the propeller. Shock transferred into the engine likely resulted in the internal damage to the engine, the disintegration of the piston skirt, and the damage to the lower cylinder skirt, necessitating the extensive rebuilding of the THOR 250 engine.(Photo of damaged prop guard cage below). With the SKYMAX's solid upper frame, I came through this incident uninjured.

These 1-1/4" fiberglass axles may be roughly 60% stronger than the previous 1" fiberglass axles, and do not have holes drilled through them close to the ends o the steel carrier tube, where maximum stress develops; the 1" axle break from hitting the badger hole might very likely have NOT occurred if heavier 1-1/4" axles had been in place, installed as shown above.


The photo below shows my Skymax quad after fabrication & installation of the new 65" I.D. prop guard cage; it is installed with SIX support struts. The spreader pieces between the double hoops extend the rear hoop well past the prop's arc.

 The photo below shows how I have my rescue parachute mounted with 3/4" cam buckle straps (from ). The blue pack which rides between my knees carries assorted tools and gear. I now use my Garmin GPSmap64s tethered to the pack's outer pocket zipper pull ring where it's easily readable while flying- it's positioned similar to using a knee board.

The PPGmeter I had installed malfunctioned after about 38 hours of service, going into a constant error message flashing mode which could not be reset; this made it unusable. So I removed it. I installed a TinyTach to go with my separate H2O temp gauge. I now have mounted a different EGT gauge. This water cooled engine runs very consistently and reliably, so if the H2O temp is within it's typical operating range of 157 to 161 degrees F, then all is OK.


Below is the DTC T100 Digital EGT Meter which I am now using; it's displaying 50 degrees F on a cool early November Colorado morning. I will only keep this gauge mounted when needed to check a change in carburetor jetting when I fly at lower elevations. It has a quick disconnect wiring connector. This meter has internal calibration and function selection programming capabilities via three internal switches; it's set up for 12V power supply now, and for reading a Type K thermocouple. I'm using the same type K thermocouple probe which I installed from the now defunct PPGmeter.

Here's a link to the meter only (with a blue LED display) for only $15.99 .You can also find a Type K thermocouple EGT probe on Ebay for under $15.00

Inexpensive DTC T100 EGT Gauge


While first flying in Arizona's Senora Desert last February, I dealt with a couple of issues. The first was re-jetting the carburetor for flying from a site at about 1100 feet ASL. (Unfortunately, my EGT gauge was malfunctioning when I first arrived there, so I read all of the meager info offered online, and found info which said that a #126 main jet was appropriate for flying at sea level &  a ways above. Unfortunately, I had by then done several extended flights at what I realized were excessive EGT temperatures. The H2O temperature gauge gives no indication that the fuel mixture was lean enough to elevate the exhaust gas temperature- with the coolant system with it's thermostat functioning as desired, the higher internal temperatures were not indicated.  It requires a working EGT gauge to have this information.

I had also read about & ordered some BLUE MAX 100:1 ratio mix oil from RPE. I taled with another ultralight engine tech about using the leaner oil mixture; he advised that, with the Polini THOR 250 running to it's power output optimum at 7500 RPM, that it would be appropriate th mix the Blue Max oil at 80:1, or in other words to use 9 ounces of oil per 5 gallons of gasoline.

When I finally got an EGT gauge functioning on my THOR 250 engine again, I found that the readings at wide open throttle were well in excess of the 1150 degrees F maximum... on climb out, I saw the EGT run up to & over 1200 degrees F before I dropped to a lower throttle setting and landed. Unfortunately, I'd done some flying at

I progressively tested a #128, and a #130 main jet in the PWK 28 carburetor, and was still seeing excessive EGT readings at full throttle until I obtained and installed a #132 main Jet.

But once I arrived at the optimum jetting, I was noticing that the engine was not running as strongly. I also checked the gearbox oil level, and found that it was not only now low, but was also very dirty / contaminated. I started checking & changing the gearbox oil much more frequently, and continued to find it getting very dirty in a short amount of flying time.

Later, when flying in the Sedona AZ area (re-jetted for that ~7000 feet ASL altitude) I continued to experience less power than should be expected from this engine & the gearbox oil continued to get dirty / contaminated quickly. I also was watching a leak on the gearbox output shaft seal throughout all of this progression & topping off the oil regularly; the shaft seal had evidently been damaged in the past when canopy lines were caught in the propeller and were wrapped tightly in the space between the propeller and the gearbox; the shaft seal was damaged in the process.

After returning to our home in South Park, Colorado, I found that the adjustable pitch prop had to be set to a lower pitch for the engine to run at full throttle in the 7400 to 7500 RPM range for optimum power and thrust production, compared to the prop pitch setting which I had been using the previous fall. This was further indication of a loss of power. This factor, combined with the rapid contamination of the gearbox oil led me to suspect that the compression rings on the piston were possibly sticking in the ring grooves, resulting in loss of compression & power, and producing exhaust blow-by past the rings back into the engine case & out through the shaft seal into the gearbox. Since the gearbox's top filler plug is vented ,as well as having a damaged / leaking gearbox output shaft seal, the exhaust gas blow-by was what was contaminating the gearbox oil so quickly. (The elevated EGT temperatures along with running the leaner Blue Max oil mixture likely were the factors which accelerated the buildup of carbon on the piston rings.

And yes, that's exactly what I found when I pulled the Cylinder & head to check the piston & rings.

 My Understanding: Sticking piston rings are the most likely cause of contaminated gearbox oil.

Excerpt from my reply to an email on this subject:  "I've experienced this after running too lean / hot in AZ last winter Internal engine pressure / gasses from ring blow-by allows some combustion products / carbon to be pushed past the inner shaft seal into the gearbox; this is how the oil becomes dirty / black. (The gear box has a venting check ball on the top aluminum cap.)

Depending on the mix oil & mixture you use, and especially when the engine has run at EGT readings above 1150 degrees F, the piston rings can begin to stick in their grooves from carbon build-up. That results in blow-by and results in that blow-by going the only place it can- into the the gearbox, with the oil getting dirty.

 After draining the coolant, You can unbolt & lift the cylinder & head (kept together in one unit) off the engine block & off the piston without disturbing the head gasket; the cylinder base gasket may be reusable- mine was.

 Remove one wrist pin retainer & slide out the wrist pin from the piston

 Carefully remove the top ring first without spreading it any more than absolutely necessary to work one end out of the groove & progressively completely out off the top of the piston. Note which side is UP and note the locator pin which is in the ring groove. Remove the lower ring next.

 You can then clean the piston, rings, & especially the ring grooves so that the rings once more slide in their grooves to full depth all the way around without dragging or hanging up. I used an Exacto razor knife to clean the ring grooves very carefully- just remove any dark deposits without removing any metal! It just takes a bit of time & patience. Clean the piston face completely too. A green Scotch-brite pad is helpful and some spray "Brake & Electrical Parts Cleaner". [Solvents alone don't remove the carbon deposits.]

 Your other alternative is to order a new piston / ring / wrist pin kit from Polini & install the new parts. With about 100 running hours on my engine, investing the time to clean up my parts was my choice at the time. I had the entire job done & the engine reassembled & ready to run in less than 4 hours. I thought that process was better than waiting three weeks for parts to get here from Italy!

 (The cylinder head's interior may be fine if left alone- just clean & lightly oil it before reassembly. It's the ring & ring groove cleaning which is important.)
 Oil & re-install the rings in their same position as when you removed, bottom ring first - same side up with the locator pins in the ring grooves observed carefully.
 Clean & oil the wrist pin & it's piston bore & install it and it's retainer clip. Re-coat the piston & rings with your 2 cycle mix oil before lowering the cylinder back down over the piston slowly, while insuring that each ring's gap is matching it's locating pin position. As each ring enters the cylinder you can gently press each ring into it's groove carefully with a non-marring tool so that it slides nicely into the descending cylinder

 Running good mix oil at 50:1 mix & insuring that the carburetor's main jet is not too lean will minimize future carbon deposits; an EGT gauge helps verify the correct jetting / mixture.
In AZ, flying at 1200 feet ASL at my landing zone, I went to a #132 main jet to get the engine / EGT running cool enough; Here in CO at 10,000 feet & above, I'm presently running a #120 main jet; my EGT at wide open throttle stays below 1115 degrees F, and runs below that a lot of the time.

Hope this information helps; I've been working on my own engines for a lot of years, so I consider this job fairly straight-forward. If you don't have the experience & skills and decide to have someone else do the work, at least you now sorta know what's involved in doing this work, or having this work done.



 I run fine on 91 octane with 10% ethanol: Shell V-Power or EXXON .***NOTE ***  {I'm NO LONGER running Blue Max Oil at 80:1 mix. I had the carbon deposit buildup in the ring grooves problem develop while running that 80:1 mix on that 'supposedly' 100:1 mix oil.  )

  Since 2019, I'm  running a 50:1 ( 2% ) mix of AMSOIL DOMINATOR mix oil;  The rings & grooves may stay cleaner longer while running the AMSOIL mix oil at the 50:1 mix - it's well recommended for doing that!

 I've also run with some of Lucas Oil's Octane Booster added when flying up here at 10,000 ASL & above. 1 Oz added per 5 gallons. 10-22-2018



During a flight on 10-20-2018, the engine suddenly quit running. I could restart the engine at idle but as soon ads I tried to open the throttle, the engine would quit running. After gliding in to a landing & getting the rig back home, I found that the jet needle had dropped through the E-clip to block fuel passage through the main jet completely. The E-clip was still in place inside the carburetor slide, but had worn & deformed so that it no longer was in the center slot of the jet needle. So I re-shaped the E-clip to once more clip into the jet needle slot well & re-assembled the carburetor. I also ordered some spare jet needle E-clips so that I can replace this worn part. I've since flown a 56 minute flight without any issues... but I plan to replace the original worn E-clip as soon as the new ones arrive.  Inspecting & replacing this E-clip after every 50 hours of flight time might be a good plan.

6-2021 UPDATE:  I've realized that, due to the design of the PWK28 carburetor, the jet needle and the Jet needle holder develop a lot of wear due to vibration. This results in increasing clearances between the jet needle & the passage through the holder, which therefore results in the engine running richer and richer over time. Changing to a smaller (lower number) main jet counteracts this to a certain extent, but the wear changes the mid-range mixture too. The bottom line is that the jet needle and jet needle holder need to be periodically replaced in the PWK28 carburetor for predictable performance. Adding the jet needle hold-down plate, as written about later on this web page, should help to minimize future wear of these parts, especially of the jet needle, due to vibration; see the June 2020 Update below.

THOR 250 KEIHN / Polini PWK28 updated main jet selection & setting notes
Last Updated 07-15-2021 by BKS

I'm flying the THOR250 with the PWK28 carburetor, in service since 2014, from a site just above 9900 ft. ASL. I'm currently running a 2 blade GSC maple blade Tech Series Prop with the urethane leading edge Inserts, 56"diameter, with the pitch set at +10.4 degrees. (I measure this at the 75% radius point with the digital pitch gauge placed on the middle of the ~flat back side of the bladeís airfoil. My peak RPM after warm-up is 7400 to 7500 RPM. I run 91 octane auto gas with AMSOIL Dominator 2 cycle mix oil 50:1

On a recent flight, once the prop uploaded a bit in flight, I noted the peak RPM at up to 7600 briefly early in the flight; once up at 14,200 to 14,600' ASL for cruising over the mosquito range's several high peaks, RPM peak was down to ~7370 to 7400 due to the reduced air density & resulting reduced power. Air temp at higher altitude likely was somewhere between 40 and 45  degrees F. [My average airspeed for this recent 57 mile flight was 35 MPH under my MacPara Charger 31 wing; My liftoff & landing glide approach airspeed with this wing & at this altitude is 27 to 28 MPH]

My newer EGT meter, at full throttle, reads up into the higher 400's C in flight [+/- 455 C peak?] with the probe mounted in the welded nut on the newer "two piece exhaust system", Ī 6" from the cylinder head.) Prop pitch was set at 11.2 to 11.4 degrees for  flying in Arizona in late April.

As noted above, main jet selection is affected by wear in the jet needle and jet needle holder, and has progressed towards smaller main jets over the years I've been running this PWK28 carburetor on this THOR 250 engine.  I'll likely replace the Needle Jet, Needle Jet Holder, and some other parts at something like a 100 flying hour interval in the future.

(Update: The previous jet needle had worn a lot before I added the hold-down brass plate; the jet needle holder was .001" larger ID than a new replacement jet needle holder. I'm still finding that my engine, prop,  & carburetor combination (1/2014 engine & carb)  now run best at this altitude (10,300 feet ASL) with a smaller main jet; I'm down to a #114 main jet to run at full-throttle at 7600 RPM, with EGT in the low 400 C range... this is a substantially smaller main jet than is expected, but the engine is running strong & the EGT reading is good, with no drop-off in full throttle RPM while running. A different carburetor may act differently.

On my previous original exhaust system (which developed repeated cracks due to the older hard-mount system) I had mounted the EGT probe only 1.25" from the cylinder head; I was seeing peak EGT readings in the low 1100's F up to ~1125 F on a previous different meter. I had come across a recommendation of a peak full-throttle EGT of between 1075 and 1150 degrees F for peak power output and fuel efficiency.... but I'm suspecting that trying to run the Blue Max "oil at 80:1 mix along with the higher EGT readings contributed to carbon buildup in the piston ring grooves. After cleaning the rings & ring grooves carefully & re-assembling the same parts, I've been using the AMSOIL Dominator mix oil at 50:1 since 2019, and am running with lower peak EGTs; this THOR250 continues to perform well for me.

The dual ignition engine uses an electronic ignition module on the secondary ignition which is, in my experience, unreliable. My first module malfunctioned / failed in under 40 hours of operating time. A second replacement electronic ignition module also failed in under 40 hours. I'm presently flying with only the magneto ignition.

Some  PWK28 carburetor adjusting issues:

<1> The original jet needle clip in my Polini PWK28 carburetor appeared to be a chrome plated brass- not spring steel. Last year, while in-flight, the clip apparently had fatigued while in service, so that the needle was allowed to DROP DOWN through the clip, totally blocking the main jet and abruptly shutting down the engine. The engine would restart at idle speed, but any attempt to open the throttle resulted in the engine killing again. I glided to a safe landing, but it could have happened over much less friendly terrain. So my recommendation is to replace that carb jet needle clip at 60 to 100 hour flight time intervals. Replacing the jet needle and jet needle holder should also be done if wear is apparent.

<2> On my 2014 Polini PWK28 carburetor, there was a air port on the top of the carb bodyís inlet throat which likely had some form of threaded plug in it. At some point, it had vibrated out, leaving an air intake leak which leaned out the top-end mixture dramatically, to the extent that I had changed the carb main jet up to a #136 to get the top end full throttle EGT to stay below the 1150 degree F limit. Once I discovered the open threaded port, I bought a 5mm threaded screw and installed it to block that air leak. Iím back to flying the #122  main jet in the Sonora Desert of AZ, and NOW flying with a #114 main jet here in Coloradoís high country.


Start your engine and let it warm up so the coolant temperature is up to ~130 degrees F. Then progressively go to 100% full open throttle, noting the peak reading for full throttle RPM. Hold it at full throttle for at least 30 to 60 seconds, watching the RPM readout; if it stays on your peak reading without dropping off at all, youíre not too lean. If it begins to drop off gradually in RPM, 10 RPM at a time, and continues to creep down as you hold full throttle, itís a prime indication that your main jet is leaner /smaller that it should be, and you should increase the main jet one step at a time until it will hold peak Full throttle RPM with no progressive sag or downward creep for 30 to 60 seconds of full-throttle running. Coolant temperature wonít tell you this; spark plug color wonít tell you this (since you likely never fly an entire flight at 100% open throttle.) Only this test will give you this vital information.

If your main jet is to rich / too large, your peak EGT will indicate that by not climbing to or above 1000 degrees F after an extended full-throttle test run. You can change jets one step at a time to get to that reading. 1050 degrees F might be a good target EGT readout on the extended 100% throttle test run- that operating range results in good power output combined with good fuel efficiency.


Base setting for the idle air mixture screw setting is 1.5 turns open from fully closed; the normal range for operation is between 1 and 2 turns open. If your adjustment ends up being outside of this range, it indicates that you may need to change the size of the idle / pilot jet.

Once the engine is running & warmed up to operating temperature, If the idle air mixture screw is closed too much after prolonged running at idle, the engine will be slow to accelerate to full RPM when the throttle is quickly opened  (= idle mixture too rich = need to open the air screw adjustment a bit) . If it drops off in RPM when the throttle is quickly opened and is slow to respond, the idle air mixture may bee too lean (= need to close the air screw adjustment a bit.)


Once you have the full throttle fuel mixture optimized, and your idle air screw adjustment correct, you are in a position to evaluate the jet needle clip setting. The stock setting is with the clip in the center of 5 slots. This setting primarily affects the mid-range fuel mixture; you may not need to change it. BUT - If, after gettng the full throttle fuel mixture (main jet selection) optimized, you climb out under full power , then drop back to an intermediate cruise speed throttle setting and find that the engine does not seem to be running as smoothly, itís time to consider whether changing the mid range mixture will improve how the engine runs. This is where the EGT meter again is the only thing which can give you this information. If, after dropping the throttle opening after a full power climb, when you reduce the throttle opening and the EGT temperature drops a lot, your mid-range mixture may be too rich, burning too cool/ burning incompletely. It might sound like itís running a bit rougher due to the too-rich mid-range mixture. You can optimize this by moving the jet needle clip UP one notch to drop the jet needle a bit deeper into the jet during mid-range throttle operation.)

IF, after doing a full-power climb, you decrease the throttle opening, and the EGT reading CLIMBS significantly, this would be an indication that your mid-range mixture is TOO LEAN, and you need to move the jet needle clip one notch lower, so that the jet needle is farther up out of the main jet, enriching the fuel mixture, and thereby cooling the fuel burn.



Remedying One Polini PWK 28 Carburetor Design Flaw

While flying my PPG quad  with it's THOR250 engine recently, I had the engine shut down while I was in the air over rough terrain. It was running fine, gauges showing nominal readouts... then, it simply quit running abruptly.

I immediately turned towards the best landing terrain within gliding distance and glided in to land on rough ground; one rear axle broke on landing, but otherwise I & the rig came out OK.

This was not my first abrupt shutdown of this engine without warning; I had experienced the same thing during another flight a couple of years before. That time, I was over better terrain, and glided in to land without any problem.

After landing & packing the wing - (I always fly with the wing packing bag in my gear pack on the quad) - I turned the ignition switch  back on & hit the starter button... the engine fired & ran at idle, but would cut out as soon as the throttle was opened at all.

Yes, same thing as the previous time, and after I had done what I thought I should.

After finally getting a rescue ride back to pick up mky vehicle & trailer, returning to where I had left the rig, replacing the broken fiberglass axle and then loading the quad back on the trailer, I drove back home to start work on this problem.

Below is a look inside the intake throat of the carburetor once I removed the air filter / silencer box. I have opened the throttle all the way, so you see that the slide is all the way up / open; you also can clearly see the main jet needle sitting bottomed in the jet tube- no circlip to keep it in the air slide. Second time this has happened to me- I've written before on this page about periodically replacing the E-clip to hopefully prevent this from occurring. I had last replaced that E-clip and installed a new jet needle about 5 or 6 months before... new jet needle & new E-clip. This shouldn't be happening -I've never seen this happen on any other air slide carburetor in the last 45 years, but it's now hapened TWICE in this Polini PWK28 carburetor within maybe three years, even after installing new parts recently..

Below is the original stock jet needle which I removed from this carburetor when installing the new jet needle and E-clip. On the left end are 5 slots for the installing the E-clip, where much of the metal of the jet needle is worn away. the wear extends down the needle shaft for nearly 40% of the jet needle's length from the top. Upon inspection, I find a great degree of wear in the air slide's jet needle hole- the newer jet needle ins now a very sloppy loose fit.

In the parts diagram below, the jet needle is part #11; the E-clip is part # 12.
In Mikuni carbs, there's a flat washer which sits just between part 31 - a plastic piece which sits over the lower end of the Part #19 air slide / throttle return spring- an sits firmly on top of the jet needle's e-clip so that the jet needle is held stationary in the air slide. In this Polini PWK28 carburetor, there is no such hold-down device to keep the jet needle held exactly in place. As designed & sold, the jet needle floats freely, able to slide up and down a significant distance through the hole in the air slide, and free to vibrate within the through-hole in the air slide.. The plastic piece which fits over the bottom of the spring has a large cavity on it.s lower side; it pins the throttle cable in place to keep it from disconnecting, but does nothing to keep the jet needle form vibrating all over the place, resulting in dangerous wear on the jet needle, the air slide jet needle mount  hole, and resulting in extensive stress, fatigue and wear on the e-clip. After a period of operation, the e-clip fatigues enough to allow the jet needle to simply drop right through, blocking the main jet fuel flow completely--- the engine can not run.

Below i an available kit of replacement parts for PWK carburetors which includes the air slide, the jet needle and two e-clips, as well as the other parts show,; the link into the Ebay listing where I ordered mine is below.

I ordered a bag of 10 e-clips from another supplier on Ebay, and installed TWO new ones on my jet needle. I'm waiting for the parts kit above to arrive, so I can replace the air slide (with the very oversize worn hole where the old jet needle had been vibrating around for too long) withj a new one.

But without fabricating a jet needle hold-down device, the same dangerous problem could rapidly develop again.

So I hunted through my collection of materials and came up with a flat brass washer , 1/2" in diameter and just under 1/16" thick which had a very small center hole. I used a #38 .106" diameter drill to drill it out where it just fits over the top end of the jet needle, where it can rest against the e-clip to hold the jet needle firmly in place when everything is assembled. The notch is cut in this hold-down plate to allow for passage of the throttle cable,  which extends into it's pocket in the lower area of the body of the air slide.

Below is a closer look at the brass hold-down plate.

Below is a view of the reassembled air slide with the brass hold-down plate just under the plastic shoe on the lower end of the air slide / throttle return spring; the jet needle with it's e-clips is now firmly held in place where it can not vibrate and wear as badly as it did previously..

UPDATE NOTE*** When replacing the jet needle, part # 35 on the carb exploded view should also be replaced, as it too suffers from wear and becomes oversize, enriching the mixture progressively; )  Unfortunately, this "Needle Jet Holder" is not included in any of the carburetor repair parts kits which I've seen. It can be ordered separately from:
Jets R Us 35A. Needle Jet Holder N413-12A00 021-581  

Once both the jet needle and the jet needle holder are replaced, you may need to readjust the size of your main jet .


A few weeks ago I had ordered a PWK28 Clone replacement carburetor from Tuning Parts Master, a seller on Ebay; the cost for the entire new carburetor was well under $30.00 !

This new carburetor uses all of the readily available jets and rebuild parts as all PWK 28 Carburetors. All I had to do was to remove the Velocity stack- type intake throat from my old Polini PWK28 carburetor (three 3mm grub screws) move the O-ring, and then mount it onto the intake of the new carb- a perfect match. New pink TYGON vent / drain lines are included with this carb from this seller.

I moved in my jet needle brass hold-down plate into the air slide before mounting the carb. I checked and found that this carb had come with a #120 main jet installed- which just happens to be close to perfect for flying up here at 10,000 feet and above. After testing I am fying with a #118 main jet here again.

For flying at lower elevations, a slightly larger main jet may need to be used; when I head back down into Arizona's low desert country next winter, I'll likely go up to about a #122 main jet for my first test run.

After setting the idle speed screw, I did some experimenting, and ended up with the idle mixture air screw set at 1-3/8 turn open from fully closed.



Replacing the propeller shaft seal without disassembly
Metric Oil Shaft Seal 26 x 36 x 7mm -THOR250 Shaft Oil Seal - For Propeller Shaft
Polini Thor Flash starter problems and fixes

EGT Meter Project 11-2018

Monitoring the exhaust gas temperature is the only way to really know that your engine is running within the optimum range as far as power, efficiency, and longevity. Its not necessary to fly with an EGT gauge active all of the time. You could optionally temporarily mount an EGT gauge to establish that you're running with your exhaust gas temperature safely within the "optimum efficiency range: (580 degrees C to 620 degrees C / 1075 degrees F to 1150 degrees F for the Polini THOR engines.)

Ounce you have your jetting optimized for a giving flying environment (elevation) you could then remove the EGT gauge until you want to change your tuning for another elevation / environment.

The devices shown below are relatively inexpensive to put together if you have tools & are somewhat handy with tools. (It's not something that a novice might do alone.) The EGT thermocouple probe installation approach is fairly simple, requiring you to simply drill one 1/4" hole in the appropriate area of the exhaust expansion chamber close to the exhaust port, and clamp the probe in place with a hose clamp. The display unit can be self-powered with a standard 9 volt battery - (you'll only need a battery snap connector, available from many sources.)

I've built mine into a housing containing the battery and an on/off switch; for really temporary service, protecting the back of the display module with some tape and mounting it with cable ties or velcro may be all you need to do for your application.

I'll be working with the type K thermocouple probe with the 2 meter long , braided metal guarded wires (shown in one of the photos & available as one of the links at the end of this short article.) [If you're able to get by with the 50 cm long wires on the probe which comes with the display module, you won't need to get the longer wired probe; I fabricated a ~5 foot long set of extension wires to optionally use with that probe while still mounting the display farther forward, in front with the other instruments.]

If you later dismount the meter & probe, you can simply clamp a 6mm screw in the EGT probe hole to seal off the exhaust using the same stainless steel clamp; reinstalling the meter later should be short work. With this mounting approach, there is no need to dismount the exhaust system from the engine to do what you need to do.

If you want to have a 6mm x 1mm pitch metric nut welded to the exhaust expansion chamber over the 1/4" hole so that this type of probe can be threaded directly into that nut, it's another optional approach. I may do that myself in the future when I next have the exhaust system dismounted from the engine.

Above: temperature display module bought on Ebay which included a type K thermocouple probe with .5 meter length wire leads.
Below: open back of the meter module; I installed mine in a plastic box along with a push-on, push-off switch and a 9 volt battery. It does not come with any back cover.

Above is a photo of a different type K thermocouple probe with 2 meter long armored wire leads; I've ordered one of these also, as the 2M length is about right for the installation on my PPG Quad.

Above: Display Module, switch, battery, and extension lead for the thermocouple probe assembled inside the ABS plastic housing- snug fit. NOTE- I do not recommend battery power for permanently installed EGT meters; wire it into the rig's 12 volt power system.

Above: the green display color seems to be more visible in bright daylight than either the blue or the red options; this module displays to .1 degree C resolution below 100 degrees, and then to single digit resolution above that.

Above: Clamp-on type mount for the short wire length EGT Probe which came with the display module; a 1/4" hole was drilled in a #28 stainless steel hose clamp, and a 6mm x 1mm thread pitch nut & a washer were used to mount the probe trough the clamp. Enameled copper wire was rigged to hold the probe tip extended in through the threaded stud. All that's required for installation into the exhaust expansion chamber is to drill a 1/4" hole where the probe tip can extend into the inside; I chose a location within about 2" of the cylinder head where the stainless steel clamp could be tightened in place. (I'll switch to the other 2 meter wire length probe later, mounting it with a similar stainless steel clamp.)

UPDATE 08-29-2021: I have replaced the EGT type K probe once since installing this EGT Gauge; the gauge continues to work well. It does consume enough power to use up 9 volt batteries fairly quickly, so this one is hard-wired into the rig's 12 volt system.

Sources for these parts:
LED DC12V Thermocouple K type -30~800℃ High Temperature Digital Thermometer | eBay
2M K Type Thermocouple Sensor Probe for Temperature Controller Xylnu 608641591781 | eBay
3.3" X 2.1" X 1.4" PROJECT BOX | All Electronics Corp
2019 Updates

After an incident which occurred by trying to 'save' a bad takeoff on rough ground on the short grass prairie, and hitting a badger hole , where the right front fiberglass axle broke, resulting in having the quad flip and the propeller contact the prop guard cage, I built up a new prop guard cage. The new prop guard cage has two hoops with an inside diameter of 65", using 6 main support strut tubes. It's assembled from 3/4" OD aluminum tubing using aluminum Tee connectors, with two pop rivets in each connection point. The cage webbing is made with tough weed-eater line routed through drilled holes.



On April 1st of 2021, I had the original (1-2014) magneto fail in-flight over rough AZ desert back country terrain.

Here are the photos of the progression of the incident, the 'recovery,and the reflections and upgrades  that followed this incident.

Above is a view looking to the north up into the canyon of the Santa Maria River, where it passes through the Arrastra Mountains. I had taken off from the PPG site near Wayside, thinking of possibly flying over to Hwy. 93 below Bagdad. I had flown to the right side of the Santa Maria River, towards the old inactive Anderson Mine site, when I began to hit turbulent air. I had just decided to turn back and fly elsewhere for the morning, and was starting a gradual turn to the right through sinking air, when the engine abruptly stopped running. Knowing that I had just crossed one of the back country ATV / UTV trails, I tightened the turn to get back over a bit better terrain. I had little time to circle, flair, and drop nicely into a narrow brushy desert wash; the photo below shows the quad exactly as it sits where I landed.  A later check of the GPS track shows that when the engine cut out and I started the turning decent, I was only 126 feet above the spot where I set it down... things happened quickly, & it was a good time to focus on doing that 'pilot stuff'!

Abo0ve & below are two more views of the desert wash where I set down the PPG quad, laying the wing out to my right as I stopped. It took a while to get the Macpara Charger wing's lines untangled form the vegetation, but there was no damage to any of the gear. I always fly with the wing bag stowed in the compact backpack that is mounted to the quad's frame between my knees, so I first packed the wing into that bag.

I had actually landed within a hundred yards of the ATV trail, so I grabbed the quad's front axle and drug / rolled it up the wash to park it just along the side of that trail. I then cut the cable ties that attach the pack to the quad frame, and prepared to start walking out to where I could get a cell phone signal, so I could call some good friends to have them come in on their UTV's to execute a 'recovery' . It's common for ATV's & UTVs to have mechanical problems while traveling the rough trails of  the"Arizona Outback", and they routinely go in and pull out a vehicle from wherever it happens to stop running.

Below is a view looking back down towards the Santa Maria river bottoms into the rough country where I landed and left the PPG Quad;  From this viewpoint, I've traveled about a mile, climbing several hundred feet of elevation, following an old mining track which the recreational riders occasionally explore these days. I'm still climbing uphill, working to get enough elevation to get a cell phone signal from the tower that's located above Alamo Lake State Park many miles to the west. The temperature is climbing, in the mid to upper 80's at this time, heading into the 90's before long.

I have my pack, which includes one 16 ounce water bottle, my cell phone, my Garmin GPSMAP64s, and my survival gear set. The primary objective at this point is to get high enough to get in cell phone contact with my friends to start the 'recovery'  crew heading my way. I'm probably 20-some miles from my landing site by back country trails at this point, and I'm not really looking forward to doing any more walking in the dry desert heat of this day than I need to. About every half hour, I'll take a modest sip of water, but I'm conserving it... ( I'm thinking that, in the future I'll maybe carry more than one bottle of water when flying this Sonora Desert back country!)

In this next photo below, I've climbed to about 800 feet elevation above where I left the PPG quad; I'm high enough to  see the Wayside area as a faint yellowish line on the ground in the far distance, and I now have a cell phone signal, and have been able to call a friend who was to arrange the rescue / recovery. I gave her precise GPS co-ordinates for where I was on the back country trail, and the GAIA GPS app on her I-phone showed one of her recent tracks as traveling that specific trail, so she knew not only where I was, but more importantly, how to travel the roads & trails to get there. One of my friends would drive my 2002 Chevy Tracker with the PPG trialer behind it on in, following two UTVs. I would get beyond the rougher terrain on the trail I was walking, to a place where the Tracker with it's trailer could be parked while all of us then went down into the rougher country to recover the PPG quad.

Below: after getting to an area where my Tracker with it's trailer could be turned around and parked, I found the best patch of shade available under the edge of this Palo Verde tree;

temperatures were into the low 90's by this time, and it was time to stay as cool as I could while my friends covered the many miles of country between.

 I updated my GPS co-ordinates with my friends, cleared some pokey ground litter, and got in under what shade I could.

A couple of hours later, Rosie, Bradley, and James got to my 'shade' spot. With the tracker parked, we rode down into the rough country below which I had walked out of earlier in the two UTVs . The only reasonable way to get rigged up for pulling my PPG quad out was to use my cam buckle straps to attsch it to the ball hitch on a receiver hitch extension to the back of Bradley's Can-Am, as seen below. The wing in it's bag is stowed in the back of Rosie's Polaris.

Above & Below; the trial out is rough, rocky, and steep in places with washouts, desert vegetation- just what these UTVs are built for!

Once out of the lower canyons & washes & back tpo where my Tracker with it's trailer was parked, I loaded the PPG quad onto it's trailer, and we headed on out. We were all looking forward to cold beverages and some food back at the Wayside Oasis Bar & Grill. However, a ouple of miles down the trail, we had to stop to change out the drive belt on Bradley's Can-Am. He had planned on doing it soon and had the replacement belt along, and always carried tools, but the time to do it had simply come upon us- there was no traveling further without doing the job. It took a while, but we all were eventually back on our way and covered the miles back to Wayside. It's great to have Friends- THANKS AGAIN!!

The following morning, I pulled the magneto cover on the THOR 250 , and this is what I found: the magneto magnet housing with the starter ring gear was riveted to the crankshaft mount inner section with six rivets on this early 2014 engine. Evidently, starting forces / kickback from the starter operation over the years had been enough to SHEAR all six rivets! This resulted in the loss of magneto generated ignition spark.

The 6 rivets which held the outer flywheel with it's ring gear and magnets to the inner shaft-mount part SHEARED OFF in flight! As a temporary fix, I cleaned up and matched the parts  back together and used a friend's wire feed welder to solidly put the magneto back together; the following morning , I was back in the air, flying with the repaired one.

The new replacement magneto assembly for the THOR 250 which I ordered is now machined from one solid piece of steel- no funky rivets!. I'm not sure when the change-over on the design of this part was made, but if you own an older THOR engine and you use the electric start system, you may want to inspect your magneto assembly carefully, and consider replacing it with the newer part. Photo below.

Below is a photo of the front axle assembly from thre Skycruiser Skymax Lowboy Quad; it's vertical pivot bolt passes down through the narrow piece of strap steel through a large hole which has only a modest amount of metal left on each side of the hole. You can see the breaks in the metal, and cracks at the end welds. I got everything back together in working position and welded everything thoroughly, building up more weld metel where the breaks had occurred.

Now, I have to admit that dragging this PPG quad out of rough back country by strapping this front axle to the extended hitch of a UTV may be a bit beyond the normal conceived service required of a PPG Quad front axle assembly... but how else was this back country recovery to be executed? It was our only option available at that remote place at that time, and it worked nicely.

Above: repaired front axle assembly, welded, painted, ready to fly.

Later in 2021, the 1" fiberglass front & rear axles, which have had to be replaced numerous times due to lengthwise stress cracks and breaks, were upgraded to 1-1/4" fiberglass axles in new chrome-moly carrier tubes; the front axle is now a one piece pass-through axle with only a single locator bolt in the center of the pass-through carrier tube.


2022 Modifications & Updates

I replaced the MOTOBAT MBTX4U 70 CCA 4.7 AH Battery with this POWER SPORT YTL5X-BS 12V 125 CCA 5 AH Battery. The new battery has the same length and width and close to the same weight, so it fits in the carrier nicely. The higher CCA rating results in better motor starting energy - the starter spins the motor faster for better electric starts. Price on the new battery was under $30, with free shipping included.

The upper motor mount bracket failed after 7 years in service, as shown in this photo. I fabricated a replacement from plate steel.

Flying from rough natural terrain results in a lot of stresses on the fiberglass axles. One of the places these stresses first show up is in fiberglass cracks running lengthwise from the axle inner end locating pins / bolts holes. After seeing some stress cracking in the 1-1/4" fiberglass axle at the right side of the rear axle carrier tube, I decided to install a one-piece pass-through rear axle. There is now only one locating bolt at the center of the rear axle carrier tube.

I replaced some failing pop rivets in the prop guard cage, and completely re-strung the entire net structure with new .065 weed eater line. Here is the quad in the trailer, with the engine doing a warm-up run.  the 145 cm diameter ground adjustable pitch E-prop out-performs any previous propeller I've used - it generates better thrust, giving shorter takeoff ground runs before achieving lift-off speed.

I recently received my new NIVIUK R-BUS 34 square meter wing. I wanted a wing which would allow me to take off at lower air speeds; the difference is substantial. My GPS data shows that I'm now lifting off at 4 to 4.5 MPH lower airspeed than I was with the Macpara Charger 31.
This R-Bus was available in the bright red, Black, and yellow "Rocket" color layout pattern, offering high visibility - something I really wanted in consideration of all of the military aircraft which share the airspace over Colorado's Central Rockies. It's not unusual to see C-130s and F15s flying over South Park- sometimes barely 400 feet AGL.

The "3D" multiple line tip steering rigging is very effective and efficient, providing nimble handling to this large wing with very moderate control line pressures.

Above & Below:
The stock Riser set routes the tip steering lines up through the middle of  the riser bundle, through a plastic thimble. When setting up to fly this wing on a PPQ quad set up with low hand point loops and upper riser guide rings on the Quad's upper frame, this routing of the tip steering lines does not work. So I ordered a pair of high quality line pulleys from Bluesky  PPG,  and mounted them just below the brake pulley rigging on the upper area of the brake riser- (Shown in the photo above.) I then fabricated  new larger tip steering toggle / handles with extension line loops, which will bring the tip steering toggles down lower to the bottom of the risers. New added sets of N52 magnetic keepers will keep the tip steering lines in a comfortable position, easily accessible- I am waiting for delivery of the magnets now. (Note: The small loop on the stock tip steering toggle would fit only a single gloved finger... I wanted comfortable room for two gloved fingers; and my aging shoulders are fond of working with the lower keeper location & longer tip steering line lengths.

Below: The new tip steering toggles with their Magnetic keepers installed in the lower position, ready to fly. A pair of N52 30mm x 20mm x 5mm were built into the new toggles, and sewn into the new keeper assemblies which are then sewn in place, wrapped around the lower A riser as shown and hand-stitched in place.




Skycruiser Manufacturing Inc - Quad Parts

Polini Parts Ė Your one stop for Polini parts here in the USA

Links To On-Line Documentation And Related Info

Groupe public Polini Thor 250 Owners | Facebook ē View forum - Paramotors. ē View topic - polini thor 250

How to Launch a Powered Paraglider Trike Capt. Kurt Fister Style - YouTube

Air - Altitude, Density and Specific Volume

factors affecting the performance of aircraft

Effects of Pressure and Density Altitude on Aircraft Performance

Warp Drive Propellers Tip Speed Calculator

VFRMAP - Digital Aeronautical Charts


Matching Prop To PPG



Scooter Parts International : KEIHIN PWK JETS

Keihin Pilot Jet Series 21 35 38 40 42 50 52 55 | eBay

CARB JETS : KEIHN PWK28 uses 6mm HEX main jets: 126 was stock ; [118 through #132 possibly useful on THOR 250]

10 piece pilot jet assortment kit - only $9.99 for all, free shipping from CA

Motorcycle Carburetor Repair Kit 28mm For PWK KEIHIN OKO Carburetor Spare Sets 6708699912300 | eBay Lucas Oil 10115 Semi-Synthetic 2-Cycle Oil - 1 Gallon Jug: Automotive

Lucas Oil Octane Booster | eBay

1pc NGK 4684 Standard BR10ES SOLID Snowmobile Spark Plug Tune Up Kit Set ls | eBay

Rick's Motorsport Electrics  Starter Motor Brushes 70_109 : Use For Poluini THOR250

Digital Red LED Temperature Meter for K Type Thermocouple EGT Sensors (12V/℃) 881314407798

  2M EGT Thermocouple K Type Temperature Probe Sensors Exhaust Gas Screw Threads 

 LED Digital Exhaust Gas Temp Gauge Car EGT EXT Turbo Diesel Petrol With Sensor

MAC PARA Technology - Charger Andy McAvin : Mac Para U.S. Distributor

Mac Para Gliders | TX Flysports

SAIL REPAIR TAPE - WHITE RIPSTOP NYLON - 2" x 25' ( #251400 ) | eBay

Ripstop repair tape, Kite repair tape | FunWithWind Kites

BlueWater 1" Climb-Spec Tubular Webbing - Package of 30 ft. -

MAX-GAIN SYSTEMS, INC. : 1" Fiberglass Axles for Quad (Now using 1-1/4" axles)

BlueSkyPPG : Bulk & Finished PPG Replacement Lines, Strobe, Assorted Parts

Airport Windsock with Reflective stripes

MotoBatt MBTX4U Battery | Quadflex AGM $36.76

Premium Quality OEM Tygon Fuel Line 1/4" ID X 3/8" OD Clear Yellow - 10 Feet | eBay

Orion Skyblazer II Aerial Signal Flare Kit | Bass Pro Shops: The Best Hunting, Fishing, Camping & Outdoor Gear

3M 3361 Silver High Temperature Stainless Steel/Acrylic Adhesive Foil Tape, 1" | eBay

APCO Mayday Bi Rescue Parachute: 47 m^2, 440# capacity

2 Pcs x AC 2A/250V 5A/120V 6 Pin DPDT Momentary Push Button Switch 6mm 1 NO 1 NC [For Dual Ignition Kill Switch]

James Weibe's THOR 250 Review: Polini Thor 250 Engine Review - The best ultralight engine we've seen

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