Flathead V8 spark plug colour variation

[Richard Farrant]

If the engine was only idling then the black on plugs would denote a rich idle mixture, which is adjustably, also choke might be partly on. When you do a plug check on racing engines, the engine is cut at running speed and knocked out of gear, brought to a halt, then plugs are checked. That way you get a true picture of the plug and can access the mixture or whether the plug needs changing to a hotter or colder type.