FAA Powerplant Written Practice Test

What does a hissing sound from the carburetor indicate during a manual propeller pull?

1. Fuel leakage.

2. Intake valve blow by.

3. Cylinder head gasket failure.

4. Blocked air intake.

The correct answer is: Intake valve blow by.

A hissing sound from the carburetor during a manual propeller pull is indicative of intake valve blow by. This phenomenon occurs when the intake valves do not seal properly, allowing air or gases to escape back through the carburetor rather than moving normally into the intake manifold. The sound results from the rapid release of pressure that occurs in this situation. In the context of engine operation, each of these components plays a crucial role in maintaining proper airflow and fuel delivery. When the intake valves fail to provide a tight seal, they disrupt the intended air-fuel mixture delivery to the engine, and as a result, a hissing noise can be heard. This condition can lead to decreased engine performance and efficiency. Other options relate to different engine issues, but they do not produce the specific hissing sound associated with intake valve failure during a manual pull. For instance, a fuel leakage would generally cause dripping or pooling of fuel rather than a hissing noise, and cylinder head gasket failure may result in overheating or oil leaks, but typically not the described sound. A blocked air intake could cause poor engine performance but would not directly manifest as a hissing sound from the carburetor.