What problem occurs if the idle mixture is set too rich?

• A. Spark plug fouling
• B. Vapor lock
• C. Increased exhaust temperature
• D. Improved fuel economy

Answer: (A)

Setting the idle mixture too rich means the engine is getting more fuel than air at idle. That excess fuel can wash the spark plug electrodes and form fuel or carbon deposits on the plugs, causing them to foul. When spark plugs are fouled, they don’t fire properly, leading to weak ignition, rough idle, and starting problems. Vapor lock isn’t tied to the air–fuel mixture at idle; it’s about fuel vapor forming in the lines and blocking flow, usually under hot, vapor-prone conditions. Increased exhaust temperature isn’t a typical result of a too-rich idle mix; a rich mixture generally cools the combustion somewhat and reduces efficiency, whereas a lean mixture can raise exhaust temperatures under certain conditions. Improved fuel economy would not occur with an excessively rich idle setting, since extra fuel is wasted.