Two questions: Does it smoke a lot when you run it slow, but then the smoke disappears when you run it wide open (or close to wide open for a while)?
Does it idle? Or atleast attempt to idle?
It sounds like you might have the same problem I have with my '91 KX 250. When I leaned out my needle, it seemed to help the situation, but not totally. Messed with everything, air filter, pre-mix, jetting, compression, etc. When I pull my pipe off, and look in the crankcase, it's loaded with oil! I am getting ready to do a leak down test to check my primary compression (Compression on the bottom side of the piston) I think I have a right side crankshaft seal leaking. If I do, I wonder if someone can tell me if I can replace it without taking the engine completely apart. They way you describe your spark plug sounds like the way mine looks. Is it hard to start the first time, but every other time that day, it starts right up?
One other thing, If I ride it really fast, and keep it above half throttle most of the time, it never loads up and runs great! Loads of Power!!!