Pelican Parts Forums - Black w/Gray Spark Plug?

Here's a good summary from the 356 registry:

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It's important to examine the condition of your spark plugs, as they can give you important diagnostic information about your engine. "Reading" plugs is a matter of experience and there is no way to describe the exact colors which should be visible at various points on the working end of the plug. Also, the plug color is largely the result of the last 2 or 3 minutes of operation, probably near idle on most engines. But there are a couple of obvious signs of which can be seen and described.

Debris filling the gap in the plug means that plug was fouled and probably not firing. What the debris is might be found below, or it might be a result of a bad plug, plug wire or terminal.

Oil on the plug means there is oil in the chamber; it should not be there. It came from around some tired rings or tired intake valve guides, or perhaps some other path. The choice on when your engine gets freshened is yours, and if the engine is not mis-firing as a result of the oiled plugs, you can use it for some longer period.

Shiny, small, metal balls on a white insulator is bad news; that's melted aluminum, usually from a piston top which is melting from excess heat. This does require more serious attention to the rest of the engine. Do not run an engine which shows this indication; it won't run much further anyhow.

A plug which is light tan to white all over also needs further investigation; that's commonly a result of a lean mixture, so the carbs should be examined. Might also be too much ignition advance or 'hot' spark plugs, and those'll be fixed soon.

Leaded gas showed even, repeatable color gradations across the spark plug face clearly related to fuel mixture. Modern gas commonly shows nearly black in good cylinders, but lighter near the gap and the tip of the insulator. If that's the worst result of getting the lead out, I'll take it. There are many more advantages.

A final word on spark plugs: Sometimes you get a bad one. Don't bother trying to clean one if all else is fine and one cylinder just lays down. Replace it. It is likely bad in some manner which is not visible or testable.

Get what info you can from the removed plugs and sort it as you will. Gap the new plugs (.025" on the NGKs), put a 'drop' (no more) of Never-Seize or equal on the threads near the start end, and take the screw-on tips off. Let me repeat: Take the screw-on tips off. It will preserve your vocabulary.

I start the plugs into the heads with the socket and extension, finger tips only, imagining the centerline angles as best I can. If one doesn't feel right, turn it backwards; there is a definite, tactile, 'click' as the two thread ends pass each other in reverse. You should then be able to finger-tip rotate it clockwise and start the plug. Start all four; go back with the ratchet or other driver and tighten them. You can feel the hollow washer crush; when it begins to 'take a set', it's done. Either refit the existing plug leads (adjusting the air cap grommets), or leave them loose for replacement.