Originally Posted by Chris Peralta

There could be a lot of reasons for a plug to fail. If you are still running very rich that could be a problem, also when you turn off the engine if you plug the exhaust that forces extra fuel into the engine and can foul a plug. When you pull the plug out look at the top of the piston and make sure there are no pitting marks on the piston. Personally I have never had luck with the McCoy plugs lasting very long. Does the plug seem to go bad in the middle of driving the engine or does it run ok untill you try to start the engine next?