Don't worry Sarge:

Everything is fine. Just remember - every cycle you have a tiny, little explosion inside the engine. Some dark cover there is because it is impossible to have perfect fuel mix all the time (remember you go slow, you go faster... full throttle...). As a result the fuel does not fully burn, leaving some residue not only on a plug but on all other parts inside the engine.

As far as I know F plug is the best one for this engine.

I am not sure (and this statement is out for discussion), but I read somewhere that it is better for the engine to have at least some castor oil in it (e.g. 80% synthetic, 20% castor).


Have a good flying!