For spark plugs appear black and sooty, what does this indicate about the fuel mixture?

• A. Engine is running lean
• B. Engine is running rich
• C. Wrong spark plug length
• D. Wrong spark plug voltage

Answer: (B)

Sooty black spark plug deposits signal a rich fuel mixture in the engine. When there’s more fuel than air, not all of it burns, so carbon builds up on the plug’s electrode and insulator, giving that characteristic black, sooty appearance. This fouling can reduce spark efficiency and lead to rough running or misfires, especially under load. If the engine were running lean, you’d expect a lighter, dry, grayish or white deposit rather than black soot. Likewise, using a spark plug with the wrong length or the wrong voltage affects ignition performance or heat range, but doesn’t produce the distinctive sooty buildup tied to excess fuel.