Customer complained about a vibration I couldn't find at first

Had a guy bring his Piper Archer into the shop last Tuesday saying he felt a weird buzz through the yoke during cruise. I checked the engine mounts, pulled the spinner, balanced the prop, everything looked fine. He's a retired airline pilot so I figured he was just being picky. On the third visit he finally mentioned it only happens above 2400 RPM with the mixture leaned out. That pointed me straight to the magneto timing. Pulled the impulse coupler, found it was off by 3 degrees. Put it back within spec and the vibration was gone. Now I ask more specific questions about when exactly the issue shows up instead of just assuming the pilot is wrong. Has anyone else had a symptom like that that turned out to be a timing issue on a Lycoming?

2 comments

2 Comments

the_sandra14h ago

Yeah, that "only happens at high RPM with it leaned" clue is a dead giveaway once you've seen it before. I'd add check the magneto timing anytime you get a weird vibe that shows up only in cruise, especially on a Lycoming with impulse couplings.

margaretj4012h ago

Yeah @the_sandra nailed it. Used to think pilots were being fussy till this one taught me different.