It's a sign of very rich running , possibly also weak spark incomplete combustion ??? Poor induction and compression ??
Check the float level and check that the float needle valve is sealing completely. A problem with either of these will cause a rich mixture resulting in a blacked out plug.
Eric27 Yes some great answers already. Probable carb but could be scored bore broken ring or valve guide. This lot will keep you busy for a while. Do keep us in the know on this one. Joe.
Could just as easily be the pilot screw needs a tweak as unless you do a plug chop (cut ignition without closing the throttle) you will just get a general trend in mixture which is usually at the low throttle opening end of things.
