Candidcameron

CandidCameron

When reading sparkplugs, particularly on a racing engine, you can look at the end of the center wire as a means of determining how close you are on ignition timing. Normally, the wire’s edges are as-sheared; but when you get really close, those sharp edges soften, just as the broken end of small glass tubing does in a bunsen flame. The late Bobby Strahlman, who for years was the tuning guru for Champion Spark Plugs, once looked at my 250’s plugs and said, “I think you might go up a quarter or a half-degree on timing.” When I asked him what he was seeing, he said the edges of my center wires weren’t softening yet.

Experienced two-stroke people look at every heat sign, such as removing pistons and looking up under the dome. There will be darkening, but not a hard, dark brown deposit that peels. Snowmobile people look at how far in on the piston crown the transfer streams keep the bare aluminum showing. Naturally, you always look for the light sandblast look on the edge of the piston—the sign of chronic light detonation.

When you see what looks like a tiny bit of gray cigarette ash on your plug, that is detonation splashing aluminum off the hottest part of the piston. Little shiny balls result from metallic oil additives.

A basis for all plug reading is “the dark ring” on the insulator. A maximum-power mixture is a bit rich from chemically correct, which means there will be some free carbon in the burnt gases. Most of the insulator is too hot for that carbon to condense on it; but at some point up the insulator, a low enough temp will be present that carbon can accumulate there, producing a dark ring. This can take some peering. As you jet down, the ring will move toward the cool end of the insulator.

—Kevin Cameron