to me it looks like you were running too lean, to rocco that's carbon build up from running too rich or not "clearing it out". Not talking for you roc, I just know that's what you think...lol

anything that is Burned (Black) in a RC motor, IMO, means it was too hot so it burned.

My Sirio .18's piston wasn't black after 8 gallons. My Ebp5 piston was light brown after 4-5 gallons. My blown 2.5's piston was black.