The primary metering system deliveres an increasing percentage of fuel as throttle position increases (phasing over the idle system) and varies fuel delivery in response to air flow and manifold vacuum.

Fuel is drawn through the metering system by the pressure drop that occurs when the incoming air flow must increase in velocity in order to pass the reduced throat areas at the main venturi and the boost venturi. This pressure-drop (or suction) is communicated to the system by the nozzle - a brass tube that opens into the inside of the booster venturi.

The fuel must pass through the restriction at the main jet and metering rod. The rod extends through the jet, reducing the amount of area available for fuel flow. If the diameter of the rod is large, then fuel flow through the jet is more restricted than if the rod were small.

After the rod and jet, the fuel enters the primary well and is drawn up the inside of the primary well tube. Sometimes this tube is called an emulsion tube. Here, the fuel is mixed with air that enters the inside of the tube through a series of small holes. The air is supplied by the main well bleed at the top of the main well. The air/fuel mixture exists from the top of the main well into a passage that leads it to discharge into the booster venturi at the nozzle.