Metering lever height variation in the WG-8 and others

by Had Robinson
updated June 22, 2019 – there are changes to this page as a result of continual testing, correction of errors, and suggestions from others.

Over the years, I have noted that Miniplane ships out new Top 80 engines with the metering lever (ML) value on the carburetor set to 1.7mm which is different than the value given in their Italian service manual (0.5mm - 0.7mm).  Extensive testing here has demonstrated that Miniplane's Italian service manual value is correct rather than the value measured in carburetors on new engines.  Why does the carburetor come from Walbro with an incorrect ML value?  It is likely due to the fact that there are two different kinds of ML diaphragms supplied by Walbro.  This was pointed out years ago by Richard Cobb.  In any case, the ML height must be adjusted to the correct value in order for the engine to perform its best.

It is important to note that Walbro corporate policy does not certify the use of any of their carburetors on engines used for aviation.  It essentially means that Miniplane and those who use their carburetors are "on their own."  If Walbro did have carburetors certified for aviation use, they would cost $500 instead of $100.  It also means we have no official help or suggestions from Walbro on how to tune their carburetors for paramotor use.  If you call Walbro and mention anything to do with aviation, they will hang up on you.

The WG-8 was not designed for the Top 80 or any other paramotor but for a Husqvarna chain saw.

Why the two different ML values?  Dave Jewell of Blue Sky PPG suggests that Miniplane may have failed to update the specifications when they changed the type of ML diaphragm used in the carburetors some years ago.  For a long time, I set ML's to the same value as on new engines.  This was a mistake.  The problems we experienced with the greater value (1.7mm) were poor throttle response and occasional stalling when suddenly going to full power.  In addition, leaning out the high speed mixture with the greater value can overheat the engine and cause permanent damage.

The measured ML value on new WG-8's has consistently been 1.7mm (but, of course, this could change at any time).  This is also the same measured value of ML's in the rebuild kits from Walbro.  This high value prevents the ML inlet needle valve from opening fully and results in a leaner fuel mixture.  At the lower value specified in the Italian Miniplane service manual, the valve will fully and deliver the needed fuel to the engine.

Why do the carburetor rebuild kits and the new carburetors come with the ML value set to 1.7mm?  The answer is that rebuild kits that come from Asia and EU have a different ML diaphragm than the kits available in North America (but this is changing).  The Asia/EU supplied diaphragm has a tang on it that fits nicely into the long end of the ML and it is about 0.38mm higher than the button type in the kits we usually get in the U.S.  As a result, the carburetors with the tang type diaphragm, need to have a ML value of 1.7mm – which is the default set by the Walbro gauge and the ML's supplied in kits both here and abroad.  The diaphragm that comes with the kits sold in North America do not have a tang but a button.  However, some US suppliers of WG8 parts supply kits with the tang type diaphragms.

Walbro WG8 metering lever diaphragms
photo courtesy of Wind-Drifter.com

I tested a newly rebuilt engine with both values.  With the high value, sudden operation of the throttle would stall the engine (it was briefly starved of fuel).  With the low value, I could hit the throttle to fully open from idle and the engine would immediately accelerate to full speed without sputtering or stalling (a properly adjusted idle circuit).  I also wondered whether running at full load for extended periods might burn up the engine if the higher value of 1.7mm was used?  This is not something I wanted to find out.  But we get a bunch of engines in here that have been burned up and I continue to think that the higher ML value contributed to their overheating problems.

Obviously, engines with the higher ML value will run but may fade when going to full power or have poorer throttle response.  There is also the risk of leaning out and overheating the engine.  For this reason, set the ML to the lower value (0.7mm) but only if your diaphragm does not have the tang.

Turkey Vulture

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