A couple of things to keep in mind. As Tom mentioned, a lean mixture will ruin a plug. Also, new plugs can be, and sometimes are bad. Most times the seal between the center element and the body of the plug leaks. A general rule of thumb that I follow is to not run up/tune your engine with the glo ignitor on the plug. Start the engine, warm it up a little, pull the glo clip off and take it from there_bob