FAA Powerplant Written Practice Test

What does the presence of heavy black soot on spark plugs suggest?

1. A lean mixture

2. Normal operation

3. A rich mixture

4. Mislabeled fuel

The correct answer is: A rich mixture

The presence of heavy black soot on spark plugs is indicative of a rich fuel-air mixture during engine operation. When the mixture is rich, meaning there is an excess of fuel in comparison to the amount of air, carbon particles do not completely burn during combustion. As a result, these unburned carbon particles accumulate and leave a residue that manifests as black soot on the spark plugs. This condition can lead to multiple issues, such as fouled spark plugs, poor engine performance, and increased emissions. Recognizing this kind of buildup helps in diagnosing carburetion adjustments or fuel system concerns to restore optimal operation. Understanding these visual cues on spark plugs is crucial for maintaining efficient engine performance and ensuring that the fuel mixture remains within the proper range for ideal combustion.