General: Fuel is metered to the high-speed circuit by the orifice provided by the metering rod jet and metering rod. It flows through the high-speed passage, where it is mixed with air entering from the bowl by way of the bleeder tube. The mixture is then discharged from the high-speed nozzle at the throat of the primary (inner) venturi.

Vacuum operated metering rod: The metering rod is vacuum operated by means of a neoprene metering rod diaphragm. The metering rod is loosely pinned to a brass sleeve which is riveted to the diaphragm. One side of the diaphragm is exposed to intake manifold vacuum by means of a passage leading to an opening in the carburetor bore, below the throttle valve. The other side of the diaphragm is acted upon by the pressure in the bowl. The manifold pressure side of the diaphragm is acted upon by a compression spring in addition to the manifold vacuum. Under low or intermediate load conditions, when intake manifold vacuum is high, the diaphragm is held back against the force of the spring. This results in the positioning of the larger (economy) step of the metering rod in the jet. When manifold vacuum drops, under high load conditions, the spring forces the diaphragm forward, positions the smaller (power) step of the mtering rod in the jet and permits the required additional fuel to flow.