Yetti, you've gotten great advice on un-dirting engines. I'm gonna give you some other advice that I think might make your modelling a little less guess-filled!

Invest in a Tachometer. They're inexpensive, and take alot of the guess work out of tuning and maintaining engines. It's not that we shoot for any particular RPM in any particular situation, but it removes the paranoia factor when we screw up. When the engine light in my car comes on, I hear all kinds of worrying noises. Most aren't related to anything but normal function. Same here. The tach can tell us if we're running rich, lean, if we're turning RPMs in the normal range for the engine, etc.

Alot of experienced guys don't use them, but I'm never sure why. I love and understand glow engines quite well, I can tune them by ear, but for me, tuning with a tach is more reliable and repeatable. Probably why I don't deadstick (OK, OK, 2 deadsticks in 3 years, then declared the glowplug dead.).

J