Which circuit controls the fuel available to the carburetor with a needle valve?

• A. Choke circuit
• B. Idle circuit
• C. Float circuit
• D. Power circuit

Answer: (C)

The fuel level inside the carburetor’s float bowl is kept constant by a float and a needle valve. As fuel enters, the float rises; when it reaches a set level, it pushes the needle valve shut, stopping more fuel from entering. This relationship controls how much fuel is available to the rest of the carburetor, which is why this is called the float circuit. The choke circuit is about air flow to aid starting, the idle circuit provides fuel at idle via the idle jet, and the power (main) circuit meters fuel at higher throttle. The needle valve specifically governs the fuel entering the bowl, so it belongs to the float circuit.