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

by Had Robinson


Running a paramotor above 4,000' MSL will cause your midrange and high end performance to suffer.  Typical of 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.

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.  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.

The best and easiest way to improve HA performance is to lean out the fuel mixture when flying at high altitudes.  Most aviation carburetors have an adjustable high speed (HS) jet.  The Walbro WG-8, on the other hand, has a fixed main jet.  Apparently, long ago too many pilots were burning up their engines by improper adjustment of the high speed needle jet so Miniplane had a fixed main jet installed in the carburetor.  In the world of 2-cycle engines, this is unique.

There are two ways to decrease the main jet size in the WG-8.  One way is to modify it and the other is to install a smaller jet.  We stock the HA jet for various engines that use the WG-8.  Contact us to purchase.  Note that the jets we supply can be new or used -- they function identically.  Walbro HA jets are difficult to obtain and have a limited production.

How to prepare for HA use

1. Learn the parts of the carburetor, if needed.  You can always refer to this diagram later.  If you have any doubts about the condition of the carburetor, rebuild it before making any modifications.  Your fuel system must be in TOP working order.  Throw out mixed fuel that is older than a few weeks.  Also, it is essential to install a CHT and tachometer on your Top 80.  It might save burning up the engine if you make a mistake somewhere....

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

3. Remove the metering diaphragm cover (#6) and the metering diaphragm.

4. 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

5. Either modify the jet or replace it. The Top 80 HA jet is a #112.  The Thor 130 HA jet is a #125.  We can custom make any sized jet, as needed.  Contact us to order.

6. Reassemble the carburetor.  Mark the surface of the metering lever diaphragm with the jet size so you will know what's in there.  Use a felt-tip pen.

Walbro WG 8 carburetor marked with the jet size 

7. To further improve the low and midrange performance, pilots can modify the throttle valve plate (#9).  We have the throttle valve plates for the WG-8. Contact us to order.

8. Finally, pilots must adjust the low speed part of the system at the altitude where they intend to fly.

9. Test fly the engine, watching the cylinder head temperature carefully.  Engines that have had the HA jet installed 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.  Changing out the main jet is done at your own risk.  Be very careful of engine temperature.