Spark plugs can be a virtual
storehouse of information, but
only if you know how to interpret the evidence. Consider this
– spark plugs are on the front line of the internal working
of the super hot combustion chamber in your engine. But the combustion
chamber is not only a source of extreme heat – there can also
be traces of oil, water, and excessive amounts of fuel invading its
In a perfect world of engine
functions, a used spark plug
would have a light gray coating of contaminates on the porcelain that
rests in the lower section of the unit (where the spark jumps the gap).
But as the saying goes “we don’t live in a perfect
world” and neither do spark plugs trying to survive in an
older, possibly worn out engine.
“Bird” is due for a fresh set of
sparklers, do it, and at the same time diagnose other engine and
carburetor functions. After a cruise of 20 or 30 miles, let the engine
cool down. Start by removing all the plugs. Lay them on the workbench
in the order of removal. Now closely examine the plugs and notice if
any have something other than a light gray coating on the business end.
Remember, a light gray tint on all the spark plugs would be the ideal
(used) spark plug condition.
If it’s got a
black sooty coating, then the
carburetor (being too rich) or the choke (staying closed) are the most
likely offenders. If the porcelain on all the plugs is extremely white
in color (almost bright white or blistered), then the fuel mixture
being delivered to them is inadequate (too lean). Check the carburetor
for restrictions in the main metering jets. Possible problems could be
undersize metering jets or a sticking fuel metering rod or rods.
And now for the
“Dear John” message you
didn’t want to hear about from your spark plug analysis. If
some of the plugs are gray or black, and others have a brown or black
caked-on material on the porcelain, then these cylinders are consuming
an excessive amount of oil, possibly caused by hard and/or brittle
valve seals (easiest to repair). Another possibility would be excessive
wear of the piston rings, especially the oil control rings –
“heaven forbid” a cracked piston. Finally, if you
notice a hint of green coloring on the porcelain, this could be the
sign of a future head gasket failure (coolant entering the cylinders).
If your plugs
are telling you any of this, then it’s
time to have a qualified professional run further tests to verify the
possibility of an engine rebuild.