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plug colours!For mixture, we don't care a bit what colour the metal parts of the plug are. Glazing and fouling and some other problems show up there, but not mixture. White is just as good as tan is just as good as grey. Bare metal colour is best. If there is unburned gas left over in the mix (meaning it's too rich, by (definition), you get soot on the centre ceramic insulator. Soot is black, period. If it's very rich, the whole insulator will be sooty. If it's somewhat rich, the tip will be clean and the soot will start partway down the insulator. The best power point is when the soot just barely doesn't appear all the way down to the base of the insulator. If the mixture is leaner than that, there's no visible change. In other words, you cannot detect leanness from reading plugs, only richness. Often the insulator will turn light grey or violet, or even orange. Those are deposits from salts in the fuel. The colour doesn't tell you anything useful. If there's a white ring around the tip of the insulator where the salts are blasted away, the timing is too advanced. If there is no white ring at all, the timing is retarded. You want about a millimetre or white around the electrode. A streak down one side of the insulator from the spark is normal -- look for a ring that goes all the way around. Black speckles on the insulator are from pinging, whether you heard it or not. If the tip of the insulator looks crumbly and porous, the heat range is too hot. If the whole plug looks dark rather than a pastel shade, it's too cold. You can tell a whole lot about an engine's state of tune from reading plugs correctly, but not from colour in particular. Dave's notes: Phil is the editor of VClassics magazine, devoted
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