Up until 2 week's ago I have run the spark plug that came in my bike from the factory, in the past hour and a half or so of run time I have fouled it and 2 others, my bike blow's some black smoke when free revving it. Once the plug is gone and the bike wont start anymore I can smell fuel pretty strong, instantly I can put in a new plug and it fires right up and runs fine, and it alway's run's what seem's to be fine right up until I shut it off and then it won't start the next time. I've checked my valves and they are in spec as well.... The only thing I can think is could it be a stuck injector that is fuel fouling my plugs? the smoking a bit started sometime a few month's ago around the same time I had no choice but to run cheap ampm gas for a ride day. Has anybody had this problem before?