Not fix themselves, but run marginally on and off.
My Pastor's scooter wouldn't start after her dropped it, and it drove me crazy. I went through every system (some twice), trying to figure out what would cause it to not run after a minor crash.
It coincidentally picked that moment to present the valves as the problem. The motor ran fine before that, and the adjusters were not loose. Weird. You can read about it here if you like:
http://www.chinariders.net/showthread.php?t=9421