FAA Powerplant Written Practice Test

In a conventional float-type carburetor, what does the fuel metering force represent?

1. The pressure in the intake manifold

2. The pressure acting on the throttle valve

3. The difference in pressure at the discharge nozzle and the float chamber

4. The fuel pressure entering the float chamber

The correct answer is: The difference in pressure at the discharge nozzle and the float chamber

In a conventional float-type carburetor, the fuel metering force is critical for ensuring the correct mixture of air and fuel entering the engine. This force specifically represents the difference in pressure at the discharge nozzle and the float chamber. When air flows through the carburetor, it creates a lower pressure at the discharge nozzle due to the Venturi effect. This pressure difference is essential because it helps to draw fuel from the float chamber into the airstream. The float chamber maintains a stable level of fuel; thus, the pressure within it remains consistent. By comparing these two pressures—the lower pressure at the nozzle and the higher pressure within the float chamber—it becomes possible to control the amount of fuel that is metered into the airflow, allowing for optimal performance and engine efficiency. Understanding this mechanism is vital for diagnosing potential issues in the fuel system, such as overly rich or lean mixtures, which could stem from problems related to the pressure dynamics within the carburetor.