Which circuit allows fuel to flow under the throttle plate when the plates are nearly closed?

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

Answer: (B)

When the throttle plates are nearly closed, the engine needs a small, steady supply of fuel to stay idle. That fuel comes from the idle circuit. It uses tiny passages and an idle jet to draw fuel into the air stream right near the throttle plates, so even with very little air flowing, the mixture is enough to keep the engine running smoothly at idle.

The choke circuit, while it also enriches the mixture, is for starting in cold conditions by restricting air, not for sustaining idle fuel flow when the plates are almost closed. The main metering circuit handles fuel delivery as the throttle opens and more air is drawn in, and the float circuit simply stores fuel in the bowl and feeds the other circuits rather than acting as the idle source itself.