FAA Powerplant Written Practice Test

During an engine ground run, which condition would indicate a mixture that is too rich?

1. Higher than normal RPM

2. Stable engine temperature

3. Rough engine operation

4. Exhaust smoke

The correct answer is: Exhaust smoke

Exhaust smoke is a clear indicator of a mixture that is too rich. When the fuel-to-air ratio is excessively rich, it means there is too much fuel being supplied compared to the amount of air. This unburnt excess fuel can lead to incomplete combustion, resulting in the emission of smoke from the exhaust. In a rich mixture scenario, the excessive fuel may not vaporize properly, causing it to exit the combustion chamber without fully burning. This can create a visible plume of smoke that is often darker in color, typically blue or black, indicating that the fuel is not being utilized efficiently. A higher than normal RPM, stable engine temperature, or rough engine operation may not directly indicate a rich mixture. Elevated RPM can suggest good fuel flow but doesn't conclusively point to richness, while stable engine temperature suggests proper operation. Rough engine operation may stem from various issues unrelated to fuel mixture, such as mechanical problems or ignition issues.