Hello; I think it might be fuel spitting back frim the carb, being blown back over the head. Try wrapping some tin foil around the head for a quick run to see if that's it. The muffler joint is another possibility. I have an old FP 40 that still does this, yet it runs just fine. I have an Irvine 40 Diesel that leaks oil from the hole the setscrew for the contra-piston lock down, is. that oil is coming past the O-ring on the contra piston, so you don't have one of those. Those two strokes are oily things, however, they oil by total loss, so the oil has to go somewhere. Exhaust diverters do a little to keep the oilyness down a bit. Since I started flying electrics, I appreciate not having to mop everything down after a days flying.