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Vibrations during full power

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May 23, 2024
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Hey guys, I'm a fairly inexperienced GA A&P. I'm more familiar with multi-engine turbo prop aircraft. I've been working on aircraft in the military for about a decade, but these reciprocating engines are gonna be the death of me. I have an issue with a 1971 Piper 140. It just got out of its 100 hour and everything was running normal (for this trainwreck of a plane). Compression was fine and we changed several spark plugs, and it has a brand new ignition harnes. Mag drop is also good. However, it's always run a little hot, never overheating, and the owner wanted us to tear into that. So after changing the oil cooler thermal bybass valve, the temp sensor, and he wanted us to change the oil cooler to solve the issue (even though I don't think that was the cause), now the plane shakes at full power. Pilots took it out on a flight and barely made it off the ground before aborting due to massive shaking. After several hours of troubleshooting on the ground, we've determined that in the run-up, at full power, it shakes hprribly. But if they lean the mixture down a little, it stops. At this point, we've drained the carb, adjusted the idle speed screw and throttle stop (both against my advice). The young instructor pilots they have flying this turd run it at full rich all the time. In my smooth monkey mechanic brain, shaking like that, that is solved by leaning the mixture has to be a full/air ratio issue. But honestly, I'm sick of dealing with it and just want to be done. I have a 100 hour on a 180 starting next week and an annual on a Cessna 310 I've gotta start dealing with for the same flight school. Any help would be appreciated. I've kinda narrowed the high temp readings down to the gauge after some troubleshooting and getting actual accurate info from the pilots. But that's a problem for tomorrow me.

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