FAA Powerplant Written Practice Test

What will be the effect of an overly lean mixture at full power on engine performance?

1. Increased power output

2. Decreased engine temperature

3. Possible engine roughness or misfire

4. Improved fuel economy

The correct answer is: Possible engine roughness or misfire

An overly lean mixture at full power can significantly impact engine performance by causing roughness or a misfire. When the air-fuel mixture becomes too lean, there is insufficient fuel in relation to the amount of air entering the combustion chamber. This can lead to incomplete combustion, resulting in unstable engine operation. In a lean condition, the fuel particles are separated by too much air, which can disrupt the steady flow of combustion processes. The increased air-to-fuel ratio may delay ignition, causing the fuel to burn slower than optimal, leading to potential misfires. When misfires occur, the engine may experience intermittent power loss, vibrations, and other irregular performance issues. Additionally, the lean mixture can lead to hot spots in the combustion chamber, exacerbating these roughness and misfire symptoms. Understanding this relationship underscores the importance of maintaining the correct fuel-air mixture to ensure smooth engine operation, especially at full power settings.