What you'll need:
What to do:
|White leaning to beige||Good WOT mixture|
|White leaning toward black||Momentary rich spot|
|Bonding agent cooked out between porcelain and center electrode, or the side electrode gets a metallic green/blue hue||Plug range too hot, or temp too high in combustion chamber, possibly too much timing|
|Heavy black flakey deposits||Too much fuel|
|Black glossy deposits||Oil|
|salt and pepper flecks/fly turds/tiny metallic balls on porcelain||Detonation. The flecks are little bits of piston|
|Fire ring (tan spot/ring)||If high up on porcelain timing is probably good. If it's near the tip you might want to try a little more timing|
|Corners rounded off center electrode (newish plugs)||lean/too much timing. Or just normal wear if the plugs are old|
|Black spiderweb traces on center electrode||Usually only on early HEI systems, a better spark box or maybe low resistance center button will fix it.|
|Pools/spots of odd colors||Fuel splashing, fuel additives. Can vary with brand. Manganese leavs a pink residue which can be a ground path and cause misfires.|
Believe what the plugs tell you, and don't try to force your ideas of what timing, etc. should be onto the engine. Just give the engine whatever it wants.
Sometimes a bad combination can give an OK looking plug (ie thermostat too cold, too much fuel, and too hot plug).
Usually on street motors you'll find one or two cylinders that read a little lean, or are a little more prone to detonation. These always need to be checked. Occasionally do them all. And when roughing in an initial tune, check all of them until you are getting close.