Hi Mike,

The "dirt" is from the exhausts leaving the pipe. I do not know what You run 2-stroke 10 cc engines on today but back in 1970th we ran them on 80 % Methanol and 20 % Castrol M 2-stroke oil (based on recin oil) and sometimes also some percentage nitro in the fuel. The planes got really dirty with that much oil but that was what we used then. I used my planes mostly from grass strips so after a whole day of flights oil and grass covered most of the plane right side and right inner wing. It took some time before they where clean and could be carried inside a car for transportation home. As I understand it there are better oils now (synthethic) and only around 10% (plus/minus some percent) of oil is needed in the methanol fuel for 2-stroke engines and that result in less "dirt" on the planes.

/Bo