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Metering lever height variation in the WG-8 and others

by Had Robinson

Over the years, I have noted that Miniplane ships out new Top 80 engines with the metering lever (ML) height on the carburetor set to a value of 1.7mm that is different than that given in their Italian service manual (0.5mm – 0.7mm).  Extensive testing here have demonstrated that Miniplane's Italian service manual value is correct rather than the value measured in carburetors on new engines from the factory.

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 they did have carburetors certified for aircraft 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.

Why is this?  Dave Jewell of Blue Sky PPG suggests that Miniplane may have failed to update the specifications when they changed 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 value on new WG-8's has consistently been 1.7mm.  This is also the same measured value of ML's in the rebuild kits from Walbro.  This high value causes the inlet needle valve to open a bit later and, as the Walbro tech page notes, results in a leaner fuel mixture.  At the lower value specified in the Italian Miniplane service manual, the valve will open sooner and deliver more fuel to the engine.

Why do the carburetor rebuild kits and the new carburetors come with the ML set to 1.7mm?  I think the answer is that rebuild kits that come from Asia and Europe have a different ML diaphragm than the kits available in North America.  This diaphragm has a tang on it that fits nicely into the long end of the ML and it is about 1.0mm higher than the tang in the kits we can get here.  As a result of the greater height, the ML needs to have a height of about 1.7mm -- which is the default height of the Walbro gauge and the ML's supplied in kits both here and abroad.  The diaphragm available on the kits sold in North America do not have a tang but a button.

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.  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 wonder if a contributing cause is the higher ML height?

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 height to the lower value (0.5mm – 0.7mm) but only if you diaphragm does not have the tang.