Spark plug reading chart

Normal
Gray brown deposits, lightly coated core nose. Gap increasing by around 0.001" per 1000 miles.
Plugs ideally suited to engine, and engine in good condition.

Carbon Fouling
Dry, black sooty deposits. Will cause weak spark and eventually misfire.
Fault: over-rich fuel mixture.
Check: carburetor mixture settings, float level and jet sizes; choke operation and cleanliness of air filter.
Plugs can be re-used after cleaning.

Oil Fouling
Wet, oily deposits. Will cause weak spark and eventually misfire.
Fault: worn bores/piston rings or valve guides; sometimes occurs (temporarily) during running-in period.
Plugs can be re-used after a thorough cleaning.

Overheating
Electrode have glazed appearance, core nose very white - few deposits.
Fault: plug overheating.
Check: plug value, ignition timing,fuel octane rating (too low) and fuel mixture (too weak).
Discard plugs and cure fault immediately.

Electrode Damage
Electrodes burned away; core nose has burned, glazed appearance.
Fault: pre-ignition.
Check: as for 'Overheating' but may be more severe.
Discard plugs and remedy fault before piston or valve damage occurs

Split Core Nose (may appear initially as a crack)
Damage is self-evident, but cracks will only show after cleaning.
Fault: pre-ignition or wrong gap-setting technique.
Check: ignition timing, cooling system, fuel octane rating(too low) and fuel mixture (too weak).
Discard plugs, rectify fault immediately.