Carburator probably needs rebuilding. You could get a kit for it or if your really cheap (like me) just clean it out real good and you'll probably end up having to make some of the gaskets that would have come with the kit.
One you have the carb cleaned if it still runs rough I would try leaning it out a little, sounds like its running rich if it's blackening plugs and sputtering.
Oh, one more thing make sure it's got spark on all the cylinders it could be running on just 3. That would make it run really bad and make almost no power.