Any motorcycle mechanics here?

Having a problem with dad's bike. Engine was completely rebuilt by a shop in town. Bike is a '71 Bonneville Triumph. Anyways we have a problem with one spark plug always fouling up. Always the same one. Bike runs fine with new plugs, then one starts fouling and the bike starts running crappy. Talked to another mechanic and he's stumped. Looking for new ideas.

[ame="http://www.youtube.com/watch?v=ivUj6vALND0"]Dad and his Bonneville Triumph - YouTube[/ame]