Re: Yanmar 2gm Fuel Injection Adjustment Screw broken off / missing - Low compression
I wouldnt necessarily panic about the valve hitting the piston. If you have a mark on the top of the piston it isnt the end of the world.
This is not to say that letting the carbon buildup get that bad isnt a bad thing but just to say that the piston hitting the valve should crush the carbon I would guess before it does other damage.
I would check that pushrod for straightness as well.
Be interesting to hear MikeHonchos reasoning. I'm not looking for an argument but he may know something or have experienced it differently.