You're going to have to look at what your plugs look like when you take them out. If they look really black and sooty, or black and oily, they're probably fouling. If they're oily looking, you are getting oil into your combustion chamber from somewhere, probably valve stem seals. This is very rare.

Now, if they're very black and sooty looking, your mixture is too rich or you're letting your engine run with the enricher in the "on" position for too long. The "too rich mixture" will mean either a carb problem or a computer setting if it's fuel injected. The bikes usually come set extremely lean from the factory so someone has either changed the jetting, there's something wrong with either the carb or computer settings.

If you allow the engine to "warm up" too long with the enricher out, it will foul spark plugs. I usually start the engine and let it run with the enricher out just until I can keep it running with the throttle. At that point, I turn the enricher off. I hope this helps. Good Luck