As a by product of combustion, carbon will become deposited on components within the combustion chamber including the spark plug. Excessive carbon deposits on the plug can cause a misfire due to deterioration of spark quality. When the spark plug reaches a certain temperature it will start to burn off the carbon and enter a self cleaning region.

Fouling will occur if plugs are operating at lower than the self-cleaning temperature of

400 degrees Celsius – 450 degrees Celsius (750 degrees Fahrenheit – 850 degrees Fahrenheit)
or
Air-fuel mixture richer than 8:1 to 10:1

This can be seen as:

Wet Fuel Fouled – Wet Black Deposit

The firing-end of the spark plug becomes saturated with fuel and its insulation ability deteriorates and misfiring occurs.

Recommendation
Check for rich air/fuel mixture. Check the entire ignition system. If condition recurs, engine overhaul may be necessary.

Carbon Fouled

Black Carbon Fouling
Carbon accumulates in large quantity and, while the firing-end of the plug is dry, its insulation is abnormally decreased. This, too, is regarded as a prime cause of misfiring.

Recommendation
Check for rich air/fuel mixture. Check the entire ignition system and cooling system (excessive cooling).

Other fouling – Oil Fouled

When the firing end of a spark plug is fouled by oil, an electrical leakage path is formed and the insulation deteriorates, consequently the available voltage from the ignition system is lowered and a spark can not jump at the spark gap.

Causes of carbon fouling

• Fuel mixture too rich
• Excessive use of choke
• Blocked air filter
• Incorrect spark plug gap setting
• Over-retarded ignition timing
• Compression loss due to imperfect cylinder-piston seal or valve seating
• Prolonged low speed driving or idling
• Too cold a spark plug fitted

Causes of oil fouling

• Lubricating oil entering into combustion chamber

Spark plug tip temperature