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ethanol and jetting

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Dana

Super Moderator
Staff member
Joined
Apr 3, 2007
Messages
12,666
Location
CT, USA
My understanding is that ethanol in fuel makes the engine run leaner. I'm wondering what other's experiences have been, I'll share mine:

The engine in my UltraStar is the venerable Cuyuna ULII-02, with the original Mikuni carburetor. The sea level (where I fly) jetting reccomendations in the Cuyuna manual are 310 from 40°-60°, 300 from 60°-80°, 290 from 80°-100°, etc. When I got the plane the original 300 jet was in the carb, and the previous owner ran on avgas, as did I at first.

The only difficulty I had at first was excessively rich running at cruise on one day when it was about 90° and very humid... I dropped the needle one notch (clip now in the top groove) and that fixed it. When the weather got colder, I switched to the 310 jet and all was well (below 40° it's too cold to fly!).

Come spring, I switched to auto gas which around here has 10% ethanol. At temperatures into the 80's it still seemed fine with the 310 jet, though I moved the needle back to the second groove. EGT has always been around 1150° and plugs look OK.

The other day, with the temperature around 90°, it seemed a bit off, EGT closer to 1100°. I switched the jet to the 300 and noticed an immediate improvement, EGT back to around 1150°.

Thus it seems going from avgas to mogas with ethanol means about 1 jet increment. Come winter, when I'm flying less, I'll go back to avgas as it keeps better.

-Dana

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