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 confines.

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.

If your “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.