Tests have been completed for the Navy by the U. S. Bureau of Standards on an instrument panel indicator which tells the airplane pilot at a glance as soon as any spark plug fails to function properly. If the plug is shown to be too hot, it is pretty certain that the surface of the insulator has been fouled. This can be corrected by reducing the richness of the fuel mixture. On the other hand, if a spark plug becomes too cold, it is because carbon has deposited on the insulator. Speeding up the engine will raise the temperature and burn off the carbon.