912 loses 500 rpm during climb

by Eric Greenwell in 912 / 914 Technical Questions 7 years ago

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Re: 912 loses 500 rpm during climb

by Eric Greenwell » 7 years ago

Eric Greenwell

5 16 5

Thanks, David, that tends to confirm my mechanic's suspicion that it is carb related.

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Re: 912 loses 500 rpm during climb

by Eric Greenwell » 7 years ago

Eric Greenwell

5 16 5

I and another pilot spent several hours a week ago, testing things and doing WOT run-ups, and the only thing we found was a speck of what looks like red rubber (like the red rubber on the hose insulation) in the left float bowl.

The run-up after that was perfect - no rough running, no RPM sagging. The next two day, I did another full power runup, a takeoff, and two 5 minute, full power climbs; the following day, I flew a two leg, two hour total trip to another airport. During the runup, climbs, and the out and return flight, the engine performed smoothly just like it did for the first 450 hours. So, I'm persuaded it's fixed, but I'll be extra cautious for the next few flights, keeping an airport in easy reach while flying.

Rubberfromcarburetor.JPG (You do not have access to download this file.)

Thank you said by: Wayne Fowler

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Re: 912 loses 500 rpm during climb

by Eric Greenwell » 6 years ago

Eric Greenwell

5 16 5

It's been a year and about 100 engine hours since I first reported this problem, during which the engine performed flawlessly - until a week ago. The same symptoms occurred right after takeoff, and I shut the throttle, easily landing on the remaining runway. During ground test the next day, the 500 rpm drop occurred; a mag check while it was running at 4500 (instead of 5000) showed normal (~100) rpm drops. The fuel is clean, the carb bowls are clean, the floats all weigh the same 3.1 grams, and I didn't find anything abnormal.

I bought some type K thermocouples which I strapped to the each of the front cylinder exhaust pipes, and ran the wires to a dual channel thermocouple meter in the cockpit. Several ground runs later, the rpm drop had not appeared. I and a very experienced 912 owner/mechanic who is helping me are very puzzled. His best guess is a valve in one of the front cylinder heads may be sticking, but since those cylinders don't have EGTs, it's difficult to say that happened.

Another 912 pilot suggested the two position propeller (1-"under power" and 2-feathered for gliding, actuated by a cockpit lever that pulls a cable) might be moving to a coarser pitch, which would slow down the engine and could produce the slight rough running I noticed. I've contacted the dealer, but not heard back yet.

Any suggestions, sensible and crazy, gladly accepted.

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912 loses 500 rpm during climb

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