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Extreme rich idle mixture - FX-3


Registered User
Arizona, USA
My Carbon Cub FX-3 with CC363i (YIO-360) has an AVStar AVX3015012-1 injection servo. This servo is similar to the AVStar LFR-NNSS5 but, according to AVStar, is not identical.

Engine has been running fine for 230 tac hours and the injection system has not been touched except for air filter change and one adjustment to idle speed and mixture (4 click leaner) to give normal rise at idle cutoff. This adjustment was 11 months go and idle speed and mixture have not been a concern since then.

A few days ago the engine stopped during landing rollout at KDVT. I was eventially able to re-start and get back to the hangar. There I ran some tests that showed the mixture was extremely rich at small throttle openings. The engine would not run under about 1,100 rpm except with mixture pulled close to idle cutoff. If the throttle was advanced with that mixture setting the mixture went extreme lean and rpm would not increase much.

Today I changed the air filter and ran another test. Exactly the same very rich idle mixure that had caused the engine stop. Adjusting 6 clicks lean made no difference. Before the air filter swap I logged a fuel flow of 2.9 gph at 950 rpm. This is about twice normal.

I have reviewed the data logs for several recent flights looking at rpm, mp, and fuel flow after engine start. The data show nothing unusual prior to the landing where the engine stopped.

Anyone with experience with these servos know of a failure mode that could cause a sudden large change in idle mixture? I have stripped and rebuilt a few carbs of various types but have no experience with these injection servos.

A piece of grit causing the fuel metering valve to hang (or not fully close) in the fuel servo will cause a very rich misture. Ran across this in a Mooney with Bendix injection, which I assume is similar. It would load up and die as soon as he throttled back to land.

-Cub Builder
I have not heard of this. I have had to adjust the idle mixture on the IO-390 in the X and NX Cub. Cub Crafters was not setting them up correctly on the early IO-390 powered airplanes.