Sounds like its running rich. Have a professional look at the carb. What you can do is get a new spark plug. Start and warm up the bike on the old spark plug.......then install the new one. Ride at full throttle for 15 minutes and then while travelling full speed hit the engine kill switch, coast to a stop and check the plug. If its black or sooty then its running rich. You can also try doing this at 1/4 throttle to check the lower rpm band for richness. If you need more info do a search for "plug chop"