Which circuit uses a metering rod or vacuum-operated jet valve that opens as manifold vacuum drops with added load?

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

Answer: (C)

When the engine experiences load, manifold vacuum drops. The device designed to respond to that drop is the power circuit, which uses a metering rod or a vacuum-operated jet valve that opens as vacuum falls. This enrichment adds extra fuel precisely when more power is needed, preventing a lean condition and maintaining performance under load. The idle circuit operates at idle with high vacuum and isn’t activated by load; the main metering circuit handles normal cruising fuel delivery, and the float circuit just maintains the fuel level.