Update:

I now run the engine on a 1:32 mix again. It runs smoother, and gains some rpm. (MVVS tuned pipe)
After a while, the black exhaust residue completely vanished. The engine needs much running in time.
Performance on a 20x8 Menz is terrific with 20 lbs measured thrust, but I busted a few on landings, so now I use the 18x10 all the time (18 lbs thrust, measured). With 18x12, flight performance is better, but the tow plane torque rolls more on power application. The 7'4" 18 lbs plane just hangs on the prop, and has towering vertical when not called upon to perform towing duties.