when i broke in my engine i ran it four stroke for a full tank "super tiger 61 ringed" after that i ran i rich two stroke for a tank and then on the third tank i slowly leaned it to where its supposed to be. its running fine now and its not that old, its probably run a gallon and a half through it but the top of the piston and the head are black black black with some sort of residue. where would that come from or what causes it. i did try a bit of homemade fuel for about 5 minutes where i was just burning 80% methly hydrate and 20% oil. engine ran fine at high speed but the transition from low to high was really sluggish so i quit. it was for no longer than 5minutes so i'm curious if thats where that black crap came from.