Forgive me if this has been covered but I am new to this forum and might have missed it. Anyways, I have a Ebay 80cc/66cc Chinese 2-stroke with less than 20 miles on it. The first day out it ran great, typical drivetrain noise. The second day out, it did ok and then started making a high pitched whine at almost any RPM. I started up a hill, trying to get the bike home, when I lost power and it felt like the clutch was engaged. After I limped the 6 miles back to the house, I took it apart to find the small bevel gear stripped 3/4 of the way up. I also noticed it was not seated completely against the crankshaft. So now I have a new gear installed and it continues to wonder on the crankshaft. The gear fits very tight and seems like the screw stays tight. Has anyone seen this before and is there a fix for it? Any help would be greatly appreciated.