NORMAL CONDITION [ WORN SPARK PLUGS ] LEAD EROSION
CARBON FOULING [ FUEL DEPOSITS ] [ LEAD FOULED ] OIL FOULED
A spark plug is a device, inserted into the combustion chamber of an engine, containing a side electrode and insulated center electrode spaced to provide a gap for firing an electrical spark to ignite air-fuel mixtures.
The high-voltage burst from the coil via the distributor is received at the spark plug's terminal and conducted down a center electrode protected by a porcelain insulator. At the bottom of the plug, which projects into the cylinder, the voltage must be powerful enough to jump a gap between the center and side electrodes through a thick atmosphere of fuel mixture. When the spark bridges the gap, it ignites the fuel in the cylinder.
The spark plugs ignite the fuel mixture in the cylinders by means of a burst of high-voltage electricity carried from the distributor. The ability of the spark to ignite the fuel is badly affected if the plugs are damaged or the spark gaps are abnormal. It is therefore important to examine used spark plugs closely and to clean them periodically. The gaps of old and new plugs should also be checked before installing them. There are three basic types of spark plug fouling: "carbon" fouling, "high speed" or "lead" fouling, and "oil/carbon" fouling.
Carbon fouling is caused from low-speed operation or a fuel mixture that is too rich. It causes missing or roughness and creates soft black soot that is easily removed. Lead fouling is caused by tetraethyl lead used in some fuels and by extended high speed operation. Lead compounds which are added to the gasoline have a bad effect on some spark plug insulators. At high temperatures, it is a good conductor and may give good results under light loads, but often fails under full loads and high combustion temperatures. In some cases, it is possible to run the engine at a speed just below the point where missing will occur; then, increase the speed (always keeping below the missing speed) to burn off the lead fouling. Lead fouling appears as a heavy, crusty formation, or as tiny globules.
The third type of fouling is found on engines that are so badly worn that excess oil reaches the combustion chamber past the piston ring, or the valve guides.
In all cases of fouling or wear, it is best to replace the plugs. To avoid having to replace plugs one at a time as they wear out, always replace the entire set, even though only one plug may be bad. Plugs should normally be replaced about every 12,000 miles.