Top of the piston will have some carbon build up. You can cleanit up but it is completly normal. I have never had an engine that did not have this no matter what fuel I use from grp to nova to os and sideinder to odonnell to byrons and now coldfusion. I run coldfusion fuel now which is one of the purest fuels I have ever seen. No color additives or other un necessary additives and there is discoloring on the top of the piston but everywhere else is clean like the engine is new.