High Altitude (HA) modification of the WG-8 carburetor

by Had Robinson


If you do the modifications correctly, you will not harm your engine but greatly improve its operation.  Of course, we can do the mods for you, if you prefer.

All engines are designed to run at or near sea level.  When they are operated at high altitudes (HA) e.g. 4,000' MSL the same volume of air contains less oxygen and so the engine requires less fuel.  The carburetors, accordingly, need to be able to have the air/fuel ratio leaned out.

To accommodate these requirements, many engine manufacturers provide carburetors that have adjustable low and high speed jets.  However, the Walbro WG-8 that is used on many of the smaller paramotors has a fixed main jet.  Apparently, the original engine that the WG-8 was designed for (a BIG chain saw) tended to overheat easily and burn up so a fixed main jet was installed in the carburetor.  In the world of 2 stroke engines, this is unique.

Thankfully, the main jet on the Walbro is easily accessible and can be changed to a smaller size which provides less fuel for the same volume of air going through the engine.  The proper sized main jet is required so that the engine runs smoothly and efficiently.  A rich mixture at full throttle may limit power output and cause the engine to run roughly.  Midrange operation, similarly, requires a leaner mixture at high altitude but having a smaller main jet has less effect.  This is because a different part of the carburetor provides fuel for midrange operation.  However, the carburetor can be modified.

I have used a HA jet at sea level but the risk of overheating was much greater.  If you weigh over 170 lb. I would not change the main jet size if you fly at sea level.  Also, I recommend increased caution when correcting midrange performance issues if operating at or near sea level.

Running a paramotor above 4,000' MSL will cause your midrange and high end performance to suffer if the carburetor is not modified.  The most annoying are the problems in the midrange and this should be corrected (see #3 below) before changing out the main jet.  Typical of full throttle HA use with the stock jet is carbon buildup inside the combustion chamber and on the spark plug, roughness, stuttering, and lower engine output.  The higher you fly, the richer the mixture and the worse are the problems.  There also will be a decrease in fuel economy.

EFI ignition  Putting an EFI system on a paramotor would solve the HA problem but such a system has other problems.  Particularly, it greatly complicates the fuel delivery and electrical systems.  The computer which meters the fuel delivery has to be programmed and sensors must be mounted all over the engine to measure things like the exhaust gas stream temperature, ambient pressure and air temperature, engine speed, throttle valve position, engine vacuum, and intake air velocity.  Think about it....  What is gained over a carburetor?  Pilots can efficiently fly over a great range of altitude with an EFI system but who flies more than a few thousand feet above launch?  If it is done, it is only for a brief period.  Kits run around $600 and would take dozens of hours to install and calibrate.  I think it is a lot of trouble for not much of a performance increase.

Changing the jet size   There are two ways to decrease the main jet size in the WG-8.  One way is to modify the jet itself (takes some time) and the other is to install a smaller jet which is a much more accurate method.  We stock the HA jet for the various engines that use the WG-8.  Contact us to purchase.  DO NOT DISPOSE OF YOUR USED JETS.

Modifying the carburetor for HA use

1. Learn the parts of the carburetor (it helps to print out the parts diagram).  Note: The values in parentheses below are the carburetor part numbers given in the diagram.  If you have any doubts about the condition of the carburetor or it has been more than a year, rebuild it before making any modifications.  Your fuel system must be in perfect working order.  Dispose of mixed fuel that is older than a month.  Oil exposed to gasoline will decay slowly and lose its lubricating properties.  Also, it is essential to install a CHT and tachometer on your engine.  It might save burning it up if you make a mistake somewhere.

2. Watch the Walbro carburetor service video if you have not already.

3. Correct the low and midrange problem first by modifying the throttle valve plate (#9).  We have the throttle valve plates for the WG-8 and WB-37, if needed.  Contact us to order.  Only if you are still having problems of too rich a mixture at or near full throttle should you modify/change the main jet.

4. To modify/change out the main jet:  Remove the metering diaphragm cover (#6) and the metering diaphragm.

5. Using the correct screwdriver (it must fit snugly in the slot on the jet), unscrew the main jet (#16) from the carburetor body.

Walbro WG-8 carburetor inside detail

6. Replace the jet (best) or modify it (not recommended because it is technical). The Top 80 HA jet is a #112 (the stock jet is a #116).

Polini supplies a #125 jet for the Thor's but for which one?  This jet is probably for the Thor 100.  I have tested the #125 jet in the Thor 130 at 4,000' MSL and it causes a severe lean condition accompanied by engine misfire at > 3/4 throttle.  This problem is easily confused with a too rich fuel mixture but is just the opposite.

It is more important to do the midrange performance modification than to decrease the main jet size.

For HA operation of the Thor 130, install a #128 jet.

We can custom make any sized jet for the WG-8, as needed.  Contact us to order.

7. Reassemble the carburetor.  Use a felt tipped pen to mark the surface of the metering lever diaphragm with the jet size so you will know what's in there.

Walbro WG 8 carburetor marked with the jet size

8. Finally, pilots must adjust the low speed system.

9. Test fly the engine, watching the cylinder head temperature carefully.  Engines that have had the HA jet modified/replaced should run 10ºC to 20ºC hotter and have noticeably higher power output.  If it starts to run over specs, shut it down immediately and determine what is wrong.  Modifying or changing out the main jet is done at your own risk.  Be very careful of engine temperature.

Be alert to a quick increase of temperature or the engine having "hiccups" in the midrange.  These are signs that you are running too lean.

Turkey Vulture