Recently had 426 motor rebuilt, been running like a champ. It has about ten hours on it, all trail stuff. Started it up yesterday, ran choke for maybe a minute then turned it off, put gear on, restarted and headed down the road. Got couple hundred yards down the trail and the bike stalled as I was slowing. Couldn't get it restarted. Pushed back to the house changed plug and fired right up. Never touched the throttle while trying to restart. What could have caused the plug to foul under normal riding conditions?