Black and Grey steaks around a Saito motor. Aluminum and oxygen make grey/black. I had this once on a Saito 150 for a few flights. Turned out to be a loose exhaust elbow. I think the elbow is steel, and it was vibrating on the aluminum muffler. By the time I found the problem the threads on the muffler were stripped. Had to purchase a new muffler. Check you exhasut elbow to head connection, your elbow to muffler, and your intake from carb to engine. Bet one of them is loose. Tighten and problem goes away, if you are not too late. Jim Tucker, Midhurst, Ontario, Canada