FAA Powerplant Written Practice Test

Before checking the idle mixture setting on a carbureted piston engine, the propeller control should be placed where?

1. In the lowest RPM setting

2. In its increased RPM setting

3. In the neutral position

4. In the full feather position

The correct answer is: In its increased RPM setting

To check the idle mixture setting on a carbureted piston engine, placing the propeller control in its increased RPM setting is essential for several reasons. When the propeller is set to a higher RPM, it ensures that the engine is operating at a speed that enhances the airflow through the carburetor. This increased airflow is crucial for accurately assessing the engine's performance during the idle mixture check. High RPM allows the engine to produce enough vacuum in the intake to adequately draw fuel through the jets. If the propeller is set at low RPM, the engine may not run smoothly, making it difficult to evaluate the proper fuel-air mixture effectively. Conversely, other positions such as neutral or full feather would not provide the necessary operational conditions for a meaningful adjustment to the idle mixture. Therefore, the appropriate setting of the propeller control contributes significantly to the verification of an optimal idle mixture in a carbureted piston engine.