Excessive carbon on spark plugs can be caused by a few different things. The most common cause is an overly rich air/fuel mixture. This can be caused by a number of things, such as a dirty air filter, a leaky fuel injector, or a carburetor that is need of adjustment. Another possible cause of excessive carbon on spark plugs is a faulty ignition system. This can be caused by a weak spark, worn out spark plugs, or a bad coil. Lastly, engine deposits can also cause excessive carbon on spark plugs. These deposits can build up on the valves, piston rings, and in the combustion chamber over time and can cause the spark plugs to fouled.