oisiaa Posted October 6 Report Posted October 6 (edited) I just bought a 1971 M20C and it's been great so far across my first 6 hours and I'm still learning the quirks. At cruise speed it seems like there is normally a vibration. I don't think it's a misfiring cylinder since I'm getting good power (~144-146 KTAS). It seems like it might be a propeller harmonic or the prop is out of balance. I've tried adjusting the prop RPM up and down in 10 RPM increments which I'd expect to help if it's a harmonic, but that doesn't really seem to fix it. I also don't notice it 100% of the time so sometimes it runs totally smooth. I tried cracking the throttle until I saw a .1" decrease and then playing with the mixture up/down slightly but that didn't seem to fix it either. That leaves me with the opinion that the prop itself might be slightly out of balance. I'll have to look in the logs, but I'm pretty sure it was just balanced not much more than 100 hours ago. Any thoughts? Edited October 6 by oisiaa
47U Posted October 6 Report Posted October 6 1 hour ago, oisiaa said: Any thoughts? Have you done an inflight mag check at cruise power settings? This might provide confidence that the mag/harness/plugs are happy. Two blade or three blade prop? Three blades may be perceived to vibe more. Make sure your tach is marked correctly for your POH/STC (as applicable). Verify the accuracy of the tach to make sure you’re not in a caution/red zone. A landing light lens which smooths the front of the cowling has been known to mediate the vibe sensation. How old are the engine mount isolators. Closely inspect the engine mount for cracks. I’m sure your gusset AD has been done, as a ‘71 C. And I agree with @Robert Hicks to have the prop dynamically balanced. 1
oisiaa Posted October 6 Author Report Posted October 6 (edited) 1 hour ago, 47U said: Have you done an inflight mag check at cruise power settings? This might provide confidence that the mag/harness/plugs are happy. Two blade or three blade prop? Three blades may be perceived to vibe more. Make sure your tach is marked correctly for your POH/STC (as applicable). Verify the accuracy of the tach to make sure you’re not in a caution/red zone. A landing light lens which smooths the front of the cowling has been known to mediate the vibe sensation. How old are the engine mount isolators. Closely inspect the engine mount for cracks. I’m sure your gusset AD has been done, as a ‘71 C. And I agree with @Robert Hicks to have the prop dynamically balanced. Thanks, I'll try a mag check next flight. It's a two bade prop. I'm typically running 2400 or 2500 rpm which (I'd hope) would be well away from any limits unless the tach is horrendously inaccurate. Already have a smooth landing light lens. The mounts isolators are almost brand new and I'm fairly sure the whole engine mount was replaced back in the 1970s per the logs....even though the last annual did the engine mount inspection AD (past owner). The dynamic rebalance might be the COA even though it was recently done (perhaps improperly). Edited October 6 by oisiaa 1
Fritz1 Posted October 6 Report Posted October 6 Before you balance prop make sure the blade play in the hub is within limits, track the blade tips to make sure blades are not bent
Denis Mexted Posted October 6 Report Posted October 6 Before you balance prop make sure the blade play in the hub is within limits, track the blade tips to make sure blades are not bent How? Pull the top plugs and swing through an arch carefully noting where the tips track. Any other guidance?
M20E for me Posted Friday at 10:39 PM Report Posted Friday at 10:39 PM On 10/6/2025 at 3:49 PM, Denis Mexted said: Before you balance prop make sure the blade play in the hub is within limits, track the blade tips to make sure blades are not bent How? Pull the top plugs and swing through an arch carefully noting where the tips track. Any other guidance? Yes, build a stack of something (boxes) right under the tip on the bottom so it is less than 1/8" and tape a piece of paper to it. Mark where the back of the first blade tip is on the paper then rotate 180 and mark where the second is. I forget what the spec should be but, if I recall correctly, it should be less than 1/8". 1
Dirty Bird Posted 6 hours ago Report Posted 6 hours ago +1 on the motor mounts. I had vibration problems and dynamically balanced the prop with not noticable improvement. I then replaced the motor mounts and it is not smooth as butter.
oisiaa Posted 5 hours ago Author Report Posted 5 hours ago Hmm, the motor mounts were just replaced in the past 2 years (before I owned it). That, coupled with a log entry of a dynamic balance makes me think the previous owner had similar vibration issues.
Andy95W Posted 35 minutes ago Report Posted 35 minutes ago It’s a long shot, but maybe check to see that the reinforcing gussets from AD 75-09-08 were installed.
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