FAA Powerplant Written Practice Test

What does heavy black soot on spark plugs indicate?

1. A lean mixture

2. A rich mixture

3. Proper air-fuel mixture

4. Excessive heat

The correct answer is: A rich mixture

Heavy black soot on spark plugs is a clear indication that the engine is running with a rich air-fuel mixture. This means that there is an excess of fuel compared to the amount of air being mixed in the combustion process. When this occurs, the unburned fuel can lead to incomplete combustion, which produces soot as a byproduct. The carbon deposits build up and manifest as heavy black soot on the spark plugs. Running rich can also lead to other issues, such as fouling of the spark plugs, reduced engine performance, and increased emissions. The ideal air-fuel mixture allows for complete combustion, which results in a cleaner burn and minimizes soot production. In contrast, if the mixture were lean, the spark plugs would typically appear white or gray, indicating that there is insufficient fuel relative to air, thus enabling efficient combustion. Proper combustion should produce minimal deposits, resulting in clean spark plugs. Excessive heat can sometimes cause different wear patterns on the spark plugs but would not typically be associated with heavy black soot.