here is the problem..........when you go to tight with head clearance detonation may occur and the visible result is the top of the piston and combustion chamber popped........in this case the damage is done and your engine will never perform as good as before until you change piston/liner and combustion chamber.........

sometimes when you're lucky the glow plug burns itself and it's a sign that the head clearance too small, or your carb setting is too lean.......then you just have to replace the glowplug and check the carb setting and head clearance.......