FAA Powerplant Written Practice Test

A hissing sound from the carburetor during manual propeller pulling indicates what?

1. Vacuum leak

2. Intake valve blow by

3. Fuel vaporization

4. Oil starvation

The correct answer is: Intake valve blow by

A hissing sound from the carburetor during manual propeller pulling indicates intake valve blow-by, which is a situation where a portion of the combustion gases escapes from the combustion chamber into the intake manifold due to an inadequate seal between the valve and its seat. This can happen if the valve margins or seats are worn, preventing a proper seal when the valve is closed. The hissing sound is the result of pressure escaping, and it can indicate a problem with the integrity of the intake system. Understanding this condition is crucial for maintaining engine performance, as any loss of compression can lead to a reduction in efficiency and power output. It's important for a mechanic to properly diagnose this sound to determine if valve adjustments, seat replacements, or other corrective actions are necessary to restore proper function.