Long ago I had a 2 stroke lose power. It was an ancient 250 single motorcycle engine and gearbox in a gokart. We warmed it over pretty extensively, but it would only go 62 mph and we wanted more. We finally ran a lot of bronze into the cylinder head (block and head were cast iron), checked it would turn over OK, and took it back to the test track. After a while it lost power, went slower and slower until it wouldn't pull itself along. I was scratching my head a bit - it still ran OK, just no power output - but my father pointed out that the cylinder head was dead loose, which had a bad effect on the compression due to extreme leakage. When we took the head off, there was a contact mark from the top of the piston, which had knocked it loose. The problem was pretty obvious after that: although the piston cleared the head when you turned it over slowly, at maximum rpm the momentum of the piston and rod at the top of the stroke stretched things a bit, so the piston hit the head. It was only over a small area, and luckily the aluminium piston was pretty strong. We ground a bit of bronze out of the head, and went right back to running it. Result: top speed was still 62 mph, didn't gain a thing.
