paragliding training center
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 which is different than the value 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. Why does the carburetor come from Walbro with an incorrect ML height? Who knows.... In any case, it must be adjusted to the correct value.
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 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.... This is what happens when American courts are run by lawyers who get rich from suing manufacturers and insurance companies and why we have virtually ZERO companies that make ultralight gliders and engines.
The WG-8 was not designed for the Top 80 or any other paramotor but for a now out of production chain saw.
the two different 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. 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. In addition, the high ML height will not allow the ML valve to open to its maximum position which also can lean out the fuel mixture.
Why do the carburetor rebuild kits and the new carburetors come with the ML 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. The Asia/EU supplied 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 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.
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 height might have 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 height to the lower value (0.5mm – 0.7mm) but only if your diaphragm does not have the tang.