Plugs can look great, and glow, but still not work right. It's funny to me how some guys sacrifice their whole program using an old plug, especially at a race or for a main. Just think how much you have invested in your outing, don't jeopardize the whole thing using an old plug. Make sure you have a good supply, and replace them if you have a problem. If the car runs better, chuck the old plug. If you find out there was some other problem, keep the old plug and use it later. my .02