WD-40 ... Very dangerous as it not only flammable when wet but is bad for electrical contacts as it leaves a residue behind that does not conduct electricity very well.
Check speaker connections first.
This is a highly ill - eagle act to do. In the old days we would put an electric drill on the drive cable, but now they make them tamper proof so they only go one way. The bezels are crimped on so you can't get inside them. I expect modern one may be electronic. How will your insurance company know what mileage you have done? I have never known an insurance cancel or fine due to mileage. You should have just disconnected it when you got close to the limit.
Buy a used one and use them for 6 months in turns. There are two or three here 23K and one with 36k
https://www.google.com/search?q=2002+yamaha+sr125+speedometer
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Question edited for clarity, full make model (what it is)'.
Question moved to Yamaha Audio.
It may be sympathetic vibration and is coming from something nearby. Put it on a folded towel or sit in the floor to test it.
https://www.google.com/search?q=YST-FSW050+bad+vibration
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Being from 1985 or there abouts, it probably needs some TLC at a Yamaha service centre.
May need new capacitors and resolder dry joints for age related issues.
Question edited for clarity, and add maker, toss in some CAPS.
Question moved to model category.
Mating surfaces not flat and true.
Reinstalled without a new gasket.
Under torqued or unevenly torqued.
No gasket goo used.
https://www.google.com/search?q=2001+Yamaha+YZ250F+valve+cover+gasket+leak
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