I have a yz400f but am posting this here bc I want to know if anyone has had this happen to them even if they dont have a yamaha or no what my problem is. The problem is my bike will foul the spark plug in a few minutes. Its valves are all in spec, new piston and rings, new water pump shaft, and whole motor was just went through. It will start up fine but has a little hesitation when you accelerate and then the spark plug fouls. Could the problem be a bad coil, spark plug cap or wire or what? Any helpful advice is appreciated!