Firstly check your jetting. Sounds like your running either too rich or too lean. (Both will turn the plug black for different reasons (And look different). If you post a picture of the plugs we'll be able to tell you what the reason is.
NEVER change the premix though - You're asking for trouble changing that (With the exception of some very old machinery - I run a series of seagull outboards on a 16:1 mix rather than original 10:1 suggested as the oil today is far superior than that of previous times).
Reset your oil premix and post a pic.
Also I believe I'm correct in saying your poppet valve should be set to "C". Try changing that and then have another look.
Is your engine hard to start to begin with? Could be an air leak which would cause the engine to run lean (Too much air getting in and "Diluting" the fuel mix. Could be as simple also as a blocked air filter causing too little air and inturn running rich.
Also silly question but you're using 2 stroke oil not 4 stroke oil in the fuel right? and small engine not outboard oil?
All this aside though we need a picture of a fouled plug to advise.
Thanks,
Pete
