I wish I knew for sure but most people say getting some dirt in the intake can cause it. In my case, once the intake valve started moving, it was fast, the surface where the valve seals with the seat was wearing and cupped. I didn't do rings or piston, I just cleaned everything up, the piston and cylinder looked new still, if I had more time I might have done more. I removed head without pulling the motor but removed the motor after I shipped my head off. Pulling the motor if pretty simple and gives you a chance to clean and lube things, plus I would not want to put a top end together in the frame.