Hi Alan, you might see if your muffler is loose, if it is then that would explain some of the residue. Is your cowl rubbing against the muffler? What I am getting at is it appears as though you are getting some kind of metal particles mixed with the exhaust causing this dark stuff. Beyond that it could be an internal problem with the engine. However not seeing it first hand I am just guessing. What isn't clear is where the liquid medium is coming from that is mixing with this stuff to cause it to coat your plane. Having run several gassers in the past I can say that I have never seen enough residue from one to cause more than small specks of black to form, and then just behind the exhaust stacks. Take a look at what I said and it very well could be a combination of both. ???? Good luck, Tom.
20 years ago