Jim Smart
September 13, 2005

If you've toyed around with Holley and Demon carburetors, the Edelbrock 1400 and Thunder Series carburetors will look foreign. They don't even function the same way. Whereas Holley and Demon carburetors have power valves and metering blocks to get fuel to the boosters, the Edelbrock 1400 and Thunder Series AVS, which are based on the Carter AFB and AVS, meter the fuel differently. The Edelbrock carburetor uses jets and metering rods that control fuel flow based on throttle position and intake manifold vacuum. Fuel flow is metered via jet and metering rod sizing. Each metering rod is tapered to where it varies fuel flow through the jet based on throttle position and manifold vacuum.

There are three basic fuel-metering systems inside the Edelbrock carburetor: the idle system, primary main-metering system (power circuit for off-idle function), and secondary main-metering system.

At idle, fuel is drawn through the idle circuit with intake manifold vacuum. Think of it as you would sucking liquid refreshment through a straw. Remember when you were a kid, and you would suck air across the tip of a straw just to be cute? That suction drew fluid up the straw, but it never quite reached your mouth. The idle circuit works, more or less, the same way. Fuel is drawn from the bowl into the idle circuit using the suction that comes from the intake stroke. The amount of fuel that passes through the idle circuit is controlled via the idle adjustment screws. Turn clockwise for less fuel, counterclockwise for more. If the engine surges at idle, one side is too lean.

The primary metering system is what meters fuel to the boosters when the throttles are open for acceleration. As the throttles are opened, additional fuel is metered through the boosters above the throttles. Fuel is drawn through the main metering circuit (also known as the power circuit) by manifold vacuum. There are two paths for fuel: the main venturi and the boost venturi. The main venturi is also known as the throttle bore. The boost venturi is where the fuel is atomized and mixed with the air.

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