What problem is caused by an excessively rich mixture at idle in a reciprocating engine?

• A. Spark plug fouling.
• B. Lean misfire.
• C. Increased fuel economy.
• D. Excessive engine rpm.

Answer: (A)

Excessively rich idle mixture leads to spark plug fouling because the excess fuel doesn’t burn completely and can carbonize on the spark plug electrodes. Those carbon deposits insulate the gap and make it harder for the spark to jump, weakening ignition and often causing a misfire or rough idle. This is why spark plug fouling is the problem associated with a too-rich mixture at idle. A lean misfire would happen with too little fuel, not too much; a rich mix actually trends toward worse fuel economy, not better; and excessive engine rpm isn’t a direct consequence of an overly rich idle.