fouling plug @ start

I HAVE A 1986 IT200s. IT IS BORED OVER FROM 66.00 TO 67.00 MM. IT HAS STOCK REEDS(NEW) A NEW PISTON AND RINGS. A NEW MIKUNI CARB W/ A 60 PILOT AND A 330 MAIN JET.IGNITION SPECS ARE BY THE BOOK. MY PROBLEM IS WHEN I GO TO START IT FOULS THE PLUG. AFTER I CHANGE IT ONCE OR TWICE IT STARTS AND IS FINE ALL DAY. I MOVED THE NEEDLE FROM THE SECOND GROOVE DOWN,UP TO THE TOP GROOVE AND DIDN'T NOTICE MUCH CHANGE. ANY HELP YOU CAN OFFER WOULD BE GREATLY APPRECIATED. THANX THE RUSSMYSTER

My `04 YZ250 does the same thing, I havent had this problem until recently; to get it going you have to crank it and rev the engine up very slowly until you hear the difference, like hitting a power band, then it will run fine; if you rev it up right when you start it, the spark plug will foul out...anyone know the problem and or solution to this??

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