I would probably try putting some oil/fuel on a rag and use something small to rub the top of the cylinder to see if it makes a difference... I wouldn't recommend scratching it off, that black is carbon buildup which is common in full size vehicles, Idk if it is "common" in our nitro engines or not.. honestly, I probably wouldn't worry about it too much, but then again I'm more used to gas engines in cars than I am the nitro engine on my rc car lol