FAA Powerplant Written Practice Test

What is one purpose of an air bleed in a float-type carburetor?

1. Increase fuel tension

2. Increase fuel density

3. Decrease fuel density and destroy surface tension

4. Prevent vapor lock in the system

The correct answer is: Decrease fuel density and destroy surface tension

In a float-type carburetor, the air bleed serves a critical function by helping to manage fuel delivery, particularly under conditions where fuel may pool or create excessive surface tension. The primary purpose of the air bleed is to decrease fuel density and reduce surface tension in the fuel, which enhances atomization as the fuel mixes with air. This process is vital for achieving efficient combustion, as finely atomized fuel allows for a more consistent and complete mixture with air, ultimately leading to smoother engine performance. The introduction of air through the bleed helps to break the surface tension of the fuel, which can otherwise hinder proper flow and atomization. By ensuring that the fuel is less dense and better atomized, the engine can achieve improved performance across different operating conditions. This functionality aligns with the overall aim of carburetion—producing an optimal air-fuel mixture for combustion. In contrast, increasing fuel tension or density would not contribute positively to carburetor function, as these effects could lead to flooding or inadequate mixing. Preventing vapor lock, while important, is more directly related to fuel system design and the management of fuel temperatures rather than the specific function of an air bleed within the carburetor itself.