as the other guys said, your running rich for some reason. That may be the jetting, but other things can contribute to it.
Check your airfilter and clean it. Repack your muffler/silencer, clogged packing can make it run rich. Get a shop manual and dial in the air screw. Unscrew the top of the carb cap and check the needle clip position. All of these things are easy to do and even if it's not the actual problem it won't hurt to get things in order.
Then if all else fails pull the carb, clean it and read about jetting it properly. While the carb is off take a look at the reeds, which may also be the culprit. It's really not that difficult if you read the links and advice people have gotten. I just did 2 bikes based on the info I got from this forum and they run great.