What does carbon fouling of a spark plug indicate?

Carbon fouling of a spark plug is primarily an indication of an overly rich fuel mixture. This condition occurs when there is an excess of fuel in relation to air in the combustion chamber, leading to incomplete combustion. As the fuel burns, unburned carbon accumulates on the spark plug, resulting in a black, sooty deposit. Such buildup can hinder the proper ignition of the air-fuel mixture, leading to misfires or difficulty starting the engine.

In contrast, a too lean mixture would typically result in different symptoms, such as overheating and a lighter color on the spark plug due to excessive burning. Electrical malfunctions would not present themselves through carbon deposits specifically, but might lead to other issues like misfires or difficulty in starting. Worn piston rings can lead to oil burning and would cause other forms of fouling, such as oil fouling, rather than carbon fouling. Thus, identifying carbon fouling as a sign of a rich mixture guides mechanics in diagnosing and adjusting the fuel system for optimal engine performance.