Which circuit uses the main venturi or booster venturi and jets to meter fuel from the discharge nozzle?

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

Answer: (B)

The main idea is that fuel metering for the engine is done where the air is flowing through the main bore. In the main metering circuit, the air moving through the main venturi (or booster venturi) creates suction that pulls fuel from the main metering jets into the discharge nozzle. The discharge nozzle then sprays that fuel into the airstream, so the amount of fuel delivered matches the engine’s air intake during normal to higher throttle. The jets size controls how much fuel can flow, and this circuit handles most of the engine’s operating range beyond idle.

This differs from the idle circuit, which meters fuel using separate passages at very low air flow, and from the float circuit, which just stores and feeds fuel to the bowl. The power circuit provides enrichment at acceleration or wide-open throttle with its own paths, so it’s not the same as the main metering circuit.