ORIGINAL: tommygun32
How did you mark your BDC? Notch the fly-wheel and put a mark on the bottom of the chasis to line it up?
If that's the case, then I would say YES, your fly-wheel has slipped and the nut wasn't screwed on tight enough.
There is no ''physical'' way for the piston to switch BDC or TDC unless you have had a drastic failure with the internal engine parts.
Good luck and let us know how it goes.
I marked it with a sharpie and lined it up with the center of the carb. I kind of already knew that it was the flywheel, because like you said, the internals would have to be in pretty bad shape for that to be the case. I just wanted wanted to get confirmation that what I was thinking is correct.
Do you guys drop the piston to BDC after each run, or is that just for break in? I'm guessing yes.