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