SPARK-PLUG MISFIRING due to lead fouling occurs when compounds of low electrical resistance are deposited on the spark-plug ceramic under low-temperature operating conditions and are subsequently raised to high temperature by operating the engine at higher speeds and loads.
Spark-plug temperature (as measured with thermocouple spark plugs) was found to be the most important variable governing misfiring. Deposit composition together with spark-plug temperature governs the shunt resistance of the spark plug. When this shunt resistance becomes sufficiently low, misfiring occurs.
Misfiring severity was shown to decrease when:
Deposits were accumulated at successively higher temperatures.
Engine conditions during deposit buildup were varied to yield higher maximum spark-plug temperatures although the average temperature was unchanged.
A phosphorus compound was added to the deposit accumulation fuel.
Fuel octane quality was increased to eliminate knock during accelerations.*