Edelbrock Carb Tuning

Over the last two issues, we have looked at carburetor tuning trickery, and what it takes to get your carburetor to perform. In our November issue, we looked at the time-proven Holley 4150/4160 series. Last month, we looked at the Demon carburetors from Barry Grant.

This month, we are going to take a close look at a time-honored name in the speed equipment business--Edelbrock. Ever since World War II ended 60 years ago, Edelbrock has steadfastly pursued the performance improvement of automobile and marine engines. It began with Vic Edelbrock, Sr., the man who founded the company and an industry. He passed this legacy of performance along to his son, Vic Edelbrock, Jr. Today, Vic's daughters, Camee and Christi, along with one of the most talented teams in the industry, have piloted the company into continuing greatness.

For many years, Edelbrock's core business had been intake manifolds. Today, Edelbrock offers cylinder heads, fuel-injection systems, carburetors, headers, mufflers, shock absorbers, struts, high-pressure lines, hoses, and a lot more.

Edelbrock produces two basic types of carburetors: the square-bore and the spread-bore. The square-bore is the carburetor most of us are familiar with. It has four fairly uniform throttle bores that sit on top of a square intake manifold opening. The Spread-Bore has two small primary bores and two large secondary bores, and is similar to the Rochester Quadra-Jet (factory on the '70-'71 429 Cobra Jet), Carter ThermoQuad (common on Chrysler, Plymouth, and Dodge musclecars), and the Autolite 4300D ('70-'74, 351C).

We're going to focus on the Edelbrock Square-Bore (known as the 1400 Series carburetor), and also look at the Thunder Series AVS carburetor, a more upscale performance atomizer from Edelbrock based on the 1400.

We give the Edelbrock Square-Bore carburetor a lot of credit for neatness. You can make significant changes to the Edelbrock carburetor without spilling fuel on the intake manifold. This means you can swap jets and metering rods, adjust floats, and replace gaskets without spilling a drop.

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.

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.

When the primary throttle bores are opened fully, the secondary throttle plates and bores are also activated, allowing double the air/fuel flow over the primaries alone. The secondaries open mechanically as the primaries reach wide-open-throttle. To prevent hesitation, Edelbrock has installed an auxiliary air valve above the secondary throttle plates. With the 1400 Series carburetors, this air valve is weighted to stay closed normally. When we open the secondary throttle plates, the inrush of air counteracts the weighting, opening slowly to prevent hesitation. The Thunder Series AVS carburetor uses an adjustable spring with this air valve, which allows you to adjust the rate that it opens.

When the secondary throttle plates are open fully, fuel is drawn to the boost venturi the same way it is on the primary side. Fuel flows through a metering jet at the bottom of the fuel bowl, rising up the secondary well-tube to the boost venturis. the primary metering circuit has metering rods and jets; the secondary metering circuit has jets only. Instead of an accelerator pump on the secondary side, fuel is drawn to the booster in liquid form through a brass nozzle, which controls hesitation.

The cold starting system is the choke--that butterfly at the top of the carburetor. As its name implies, the choke cuts off the air supply through the carburetor, allowing more fuel to flow through the idle and main metering circuits when the engine is cold. As the engine warms, a thermostatic, bimetallic coil in the choke assembly (automatic chokes only) expands, pulling the choke off. This coil is heated with either an electric heating element or exhaust manifold heat drawn up from the manifold or header below. Intake manifold vacuum is what draws the heat from the manifold or header to the coil.