Which circuit provides additional fuel for cold start-up with a bi-metallic spring?

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

Answer: (B)

Cold starting needs a richer fuel–air mixture, and the choke circuit is what provides that enrichment. The choke uses a bi-metallic spring to hold the choke plate closed when the engine is cold, which reduces the amount of air entering the carburetor and effectively richens the mixture. As the engine warms, the bi-metallic spring bends and gradually opens the choke, returning the mixture toward normal. This mechanism is separate from the idle, main metering, or power circuits, which regulate fuel under normal operating conditions and don’t rely on a temperature-sensitive spring to assist cold starts.