Which circuit enriches the mixture for cold starting using a bi-metallic spring?

• A. Choke circuit
• B. Float circuit
• C. Idle circuit
• D. Main metering circuit

Answer: (A)

The choke circuit is responsible for enriching the mixture during cold starts. It uses a bi-metallic spring to move a choke valve that restricts incoming air. When the engine is cold, the choke remains more closed, increasing the fuel-to-air ratio so the engine starts more easily. As the engine warms, the bi-metallic strip heats up and the choke gradually opens, leaning the mixture back toward normal operation. The float circuit controls fuel level in the bowl, the idle circuit supplies fuel at idle, and the main metering circuit handles fuel at higher throttle—none of these provide the cold-start enrichment the choke offers.