Jim Smart
September 13, 2005

Fuel is metered through the main metering jets by main metering rods that control flow based on throttle position and manifold vacuum. the metering rod is progressive. It is narrow at its tip and becomes wider along its length. At low load (high manifold vacuum, throttles closed), the largest part of the metering rod fills the main metering jet, allowing very little fuel to flow. As the throttle is opened, the metering rod is moved via vacuum and mechanical movement. The narrow end of the rod transitions through the jet, allowing more fuel to flow through the main metering circuit and the boosters. The boosters mix the air and fuel above the throttle plates. When we goose the throttle, fuel can be seen atomizing (misting or vaporizing) at the boosters.

The main fuel metering is adjusted by changing the jet size and/or metering rod size, depending on how much change is desired in the fuel delivery. Sometimes, we change only the jet. Other times, we change both the jet and the metering rod. Your Edelbrock Performer Series carburetor owner's manual has all the details to help guide you through jet and metering rod changes.

During normal driving, the carburetor operates on the primary throttle bores, boosters, and main metering circuit. At idle, we know we are drawing fuel through the idle circuit. As the throttle is opened, we are transitioning from the idle circuit to the main metering circuit. To get there, fuel has to keep flowing between the idle circuit and power circuit. This is where the transition circuit comes into play. Fuel has to flow through the transition circuit to keep the engine from stumbling. The accelerator pump also provides a shot of raw fuel to keep the fire lit.

The accelerator pump sprays liquid fuel into the throttle bore as the throttle is opened to help prevent hesitation. The amount of fuel sprayed into the bores depends on the pump discharge nozzle size, and how aggressive the pump shot stroke is. The pump shot is changed by adjusting the location of the pump rod. When the stroke is lengthened, we increase the amount of fuel sprayed into the bores by the accelerator pump. Decrease the stroke and you will have less fuel.

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