Not experienced this before. My engine suddenly started making an alarming whine. I finally decided it was the drive train to the magdyno. Slackening the strap bolt temporarily relieved it, this having allowed the unit to realign? I am guessing it is either too tight, slack or out of alignment. Tweaking the unit with it running did not make any obvious improvement. Anyone familiar with this/ Which might be most likely and possible solutions? Shim under?
Usually, whine noises happen due to the gears being too tight together,
But it usually doesn’t happen on its own..
Can it be a bearing on the dynamo or magneto?
You can try and pull the dynamo out and see if you continue getting this sound,
And also check its bearings while its out.
Noam and Dave i think have got it right, a bit of careful examination is needed here. I had the same problem once when i used to run my M20 as daily transport. I was running a short 45W dynamo with the small bronze bush at the brush side. This had run dry and must have grabbed and the slipping clutch on the magneto had kicked in. Once i had got it home it freed but on inspection it was obvious what the problem was. You can also get a whine from the gear train but it will not come on its own unless the magneto is slack and it has moved out of line.
What i do when fitting and re timing a magneto on a M20 is once timed and the strap fully tightend i check for a small amount of backlash on the magneto drive gear rotating the gear through 360 degrees. If this is not correct try resetting the magneto, if still no backlash you will need to fit shims under the magneto between the magneto base and the crankcase.
