Spark plug non-foulers are devices that are installed in the spark plug holes of an engine to prevent fouling of the plugs. They work by creating a small air gap between the tip of the plug and the bottom of the hole, which allows any deposits that may build up on the plug to be burned off before they can cause problems. Non-foulers are especially useful in engines that are subject to frequent idling or short-duration operation, such as race cars or boats.