Dude HPI APollo is absolutely correct, the sleeve is tapered and expands when it gets hot and you acually loose compression if you overheat your motor, then the sleeve contracts again as it cools and you will notice compression again. I understand you have been in this hobby for a year now, but I am 40 and have been involed with models since I was a kid, and I go here for help because I still learn something here everyday, just give some veterans the benefit of the dought and you will be happier when you model runs right. Black ontop of you piston is normal, its burnt castor. try closing your bottom and top end a little then try again. there is nothing worse than having a motor run bad. Well I can think of a few other things but thats another topic...