What do you think might be causing a black residue in the exhaust of a rather new .90 sized 2-stroke. Broken in right, now running Omega Coolpower 15%, and a 13-6 prop. Engine starts fine, runs fine, idles fine, is reall OK, I just worry a little about her.

Overheard at the field (about a bipe): "It glides fine, it just doesn't move forward"