Certainly check for anything holding the plunger open but this sounds exactly like the history of my 91FZ. The problem may be due to ring blow-by screwing up the regulator. Dave Shadel of Performance Products in the US suggested that this might be the problem with mine. I struggled with it for near a year until sending to him - where it is now. There is a modification they do that consists of drilling holes down from the top of the piston to the back of the ring land. Combustion gasses push the ring tight. You might want to post your problem in the YS engine support forum and see what he says.