If the plug is black its running too rich. Try to lean it out with the mixture screw on the front bottom of the carb. If that doesn't help then the jets are too big or your exhaust has issues, also check your intake and air filter, make sure its breathing good. Running too rich either means she ain't breathing right or the carb is dumping in too much fuel.
If messing with it don't get you anywhere buy a new carb. For the cost of a new carb off eBay you could get a rebuild kit but then your still stuck with an old carb that is worn. Sometimes it just easier to replace!
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1984 Honda ATC200ES "Big Red"
1982 ATC200E "Hondie"
1988 TRX300FW "Project Quad" Still in progress....