springcreek

My Feedback: (3)

I was doing final ground runs this morning on my G-38 powered H9 Super Cub and had a problem I have never experienced before. The engine start and ran fine, however, at low throttle, it would continue to run at about 50% power. I tried all kinds of mixture settings to no avail. I also uncowled the engine and checked and re-checked the throttle linkage. The servo is pulling the throttle arm on the carb completely closed but the rpm continues to run high. I even took the carb apart and gave it a thorough blowing out with carb cleaner and visually checked the diaphragms for leaks. None found. The float needle was not stuck. I simply could find nothing wrong. I put the carb back together and remounted it on the engine and had the same problem.
This engine had run fine on another airplane that was destroyed in a botched inverted flat spin recovery. Now, it won't idle down and that is all that is keeping me from flying the new Cub. Any ideas, suggestions, etc, would be greatly appreciated! This hobby is supposed to be fun, but my present mood would not reflect that.
Thanks in advance.

branded

My Feedback: (9)

I think you have a crankcase air leak. I used to have a G38 that had a tap for a BB msoke pump, and when the hose would come off, the engine would not come down on idle.

Make sure that your crankcase is sealed, and if you have a tap for smoke that you're not using, then you have to seal it off.....

canman

Branded is absolutely correct - you are leaking air into the case - check to see if the carb is tight or if you have one of those new twist mounts - they seem to develop cracks easily.

My Sachs behaved the same way last month when a screw came out of the case - this made it run very lean and caused some cylinder and piston damage.

Dave

springcreek

My Feedback: (3)

Thanks for the input, gentlemen. I'll check that out today. Sounds like something that could definitely cause my problem. Unfortunately, we are experiencing a strong thunderstorm weather pattern today and flying is out of the question. Sigh, will have to wait for another day. I'll let you know if I find the problem.

Antique

My Feedback: (4)

Another place for a leak is the 4 screws that hold the engine to the mount--they go all the way through to the inside of the case....

ltriska

Once had similar idle problems with a quadra 40 after an untimley landing (ie crash) found a hairline crack at the carb mount. A lttle j b weld took care of it .

springcreek

My Feedback: (3)

Thanks all for your input. I think I found the problem. I removed the carb again and this time also removed the nylon spacer block between the carb and the cylinder. The screws that hold this block on were bent (post crash damage that I had not seen) and the gasket between it and the cylinder was damaged, quite possibly causing an air leak there. I will install replacement parts tomorrow and hopefully get a test flight or two in before our club meeting.

springcreek

My Feedback: (3)

A new gasket and 2 new mount screws solved the problem. The engine is running great again and responding appropriately. Thanks again for your help.