FAA Powerplant Written Practice Test 2026 – Complete Exam Prep

A common normal characteristic of spark plug usage with a rich mixture is heavy black soot buildup. When the mixture of air and fuel in an engine is rich, meaning there is more fuel than optimal for combustion, it leads to incomplete combustion. This results in unburned carbon particles accumulating on the spark plug.

As this soot builds up, it can lead to various issues, including fouling of the spark plugs which can affect engine performance, since a fouled spark plug may not ignite the air-fuel mixture effectively. The presence of heavy black soot on the spark plugs is an indicator that the engine is operating with a rich mixture, which is why this option is correct.

In contrast, other characteristics such as a bright white color typically signify a lean mixture, overheating is associated with too lean a mixture or other issues, and running too lean is contrary to a rich mixture where there’s an excess of fuel. Thus, the identification of heavy black soot as a sign of a rich mixture is essential for understanding engine performance and maintenance.