How many hours of the engine? Almost sound like the the piston clearance is too tight. Check side wall of cylinder on the exhaust side for scoring. Scorring is normally due to a cheap oil/overheat situation. As far as the black cylinder, I have never seen anything like that before, when you pull the jug off all should be shinny except piston top and head, maybe a bit of slight darkning of the exhaust side. Let us know. Curious now !