have 2 speed wnginesand I run super clean no additives cold fusion 30%. The rest of the engine looks like it is still new but on the top of the piston there is some carbon. This is wher the boom happens so there will always be some blackening. I have cleaned it off on engines before but if the engine is running well and the bearings are good I leave it alone. If I have to tear it down then I'll clean the top of the piston off otherwise it's not a prob. With 30% you want to use the extra shim. It's as simple as that.