Which statement best describes the meaning of black spark plugs that are visibly sooty?

• A. Engine is running rich
• B. Engine is running lean
• C. Spark plug gap is incorrect
• D. Ignition timing is off

Answer: (A)

Sparks plugs that are black and sooty point to an excess of fuel relative to air in the combustion chamber. This carbon buildup forms because the extra fuel isn’t fully burned, leaving soot on the plug tip. That’s the hallmark of a rich air–fuel mixture, which is why the statement that the engine is running rich is the best description. If the engine were lean, you’d expect lighter, chalky deposits and hotter combustion; timing issues or an incorrect plug gap tend to cause misfires or weak sparks without the characteristic soot buildup.