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

by Had Robinson

Introduction

CAUTION: DO NOT MAKE ANY CHANGES TO THE CARBURETOR UNLESS YOU HAVE INSTALLED A CHT AND LEARN HOW TO READ IT.  IT IS EASY TO BURN UP THE ENGINE.  BE EXTREMELY CAREFUL.  NOTE: MODIFICATIONS TO THE CARBURETOR WILL VOID ANY WARRANTY BY THE ENGINE MANUFACTURER AND THEIR U.S. DISTRIBUTORS.  USE ONLY APPROVED OILS AND ETHANOL-FREE GASOLINE OR AVGAS IF YOU WANT YOUR ENGINE TO RUN THE BEST AND LAST LONGER.

I have used the HA modification at sea level but the risk of overheating is much greater.  If you weigh over 170 lb. I would not change the main jet size if you fly at sea level.  Use increased caution when correcting midrange midrange issues.

Running a paramotor above 4,000' MSL will cause your midrange and high end performance to suffer.  The most annoying are the problems in the midrange and this modification should always be done 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.

Midrange modification first   Always do the midrange modification and THEN, if you are still having problems at or near full throttle, change out the jet.  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.

Changing the jet size   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, must be ordered in quantity, and have a limited production.  DO NOT DISPOSE OF YOUR USED JETS!

Modifying the carburetor for HA use

1. Learn the parts of the carburetor and print out 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. NOTE: It is more important to 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 spares are needed.  Contact us to order.  Only if you are still having problems at or near full throttle should you modify 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. Either modify the jet or replace it. The Top 80 HA jet is a #112 (the stock jet is a #116).

Polini supplies a #125 jet for the WG-8 (the stock jet is a #130), apparently for high altitude use in the Thor 100 and 130.  We have tested this jet in the Thor 130 at 4,000' MSL and it causes a 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.  DO NOT USE THIS JET UNLESS YOU ARE FLYING AT 7,000' MLS OR MORE!  It is much more important to do the midrange performance modification than to decrease the main jet size.  For HA use for the Thor models 100-130, install a #128 jet.  We can custom make any sized jet, as needed.  Contact us to order.

7. 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 

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.

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