912 idle jets

[avidflyer]

Hi all, I've been fighting with a 912 80 HP engine in my project that has been keeping me from finishing it and getting the AnP to do the inspection. When I got the engine, it had some ignition issues that I solved. Broken wires and 1 bad module. Finished the project, and could not get the engine to idle smooth no matter what. Spit fuel out the overflow tubes. Even swapped carbs from a 912 that was able to idle smooth. No joy. Lowest compression is 78/80. Last week some one on another site said to try # 30 idle jets, it worked for them when they had the same issues. Got the jets and low and behold, I have a pretty smooth idle now. May have to tweak the sync, just a bit, but no fuel going out the over flow tubes once. So now I'm wondering, why does this engine want the leaner idle jets? Anyone else running the leaner jets? If it wanted a richer jet, I would figure an air leak, but what about if you need to lean it down. Maybe I shouldn't look a gift horse in the mouth, as the saying goes. Any comments? JImChuk

[Dusty]

An aircraft at our field changed to the later style fuel pump and gained a rough idle (not as bad as yours ) but all the symptoms of being rich. I stripped the pump and collapsed the spring 12 mm(approximately 1 psi) his idle is now good again! The floats and levels were within spec.

[avidflyer]

I do have the newer style fuel pump, also have a return line that goes back to the header tank. .040" orifice in that return line. Also checked the pressure on the line going into the carb, and showed 3 lbs at idle. Was going to rig a temporary gravity flow fuel supply to the carbs to see if that made any difference, but didn't get to it. ??? Other 912 has the old style fuel pump. Floats were weighed as well with electronic scale and my reloading scale that measures in grains. Both said the floats were light enough. If memory is correct, the heaviest one was 3.1, lightest one was 2.8 grams. JImChuk

[Dusty]

Yup fuel pressure shouldn't be your problem with the return line. Without a return the new pumps push 4.7 pounds which seems to cause a few issues.