What is spark plug fouling?

A It is a phenomenon by which carbon accumulated at the firing end causes electrical leakage that leads to misfiring.

As the high voltage generated by the ignition coil leaks away through the carbon, misfiring may occur and cause running and starting difficulties.

Fouling

Insulation resistance vs. voltage generated by ignition coil

As carbon builds up, the insulation resistance of the spark plug drops and the voltage generated by the ignition coil is reduced. When the generated voltage becomes lower than the required voltage of a spark plug (the voltage needed to cause sparks at the spark gap), sparking is suppressed and misfiring occurs.

Causes of fouling and corrective actions