|
|
| |
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.
|
|
|
|
|
