Jim Smart
December 1, 2006

There's no end to the theories regarding carburetion, performance, and fuel efficiency. Just about everyone has a belief they stand by when it comes to carburetors. Fatten the mixture. Lean the mixture. Use this jet. Use that jet. Swap the power valve. Change the boosters. Use this gasket. Use that gasket. Raise the float level. Lower the float level. Use vacuum secondaries. Use mechanical secondaries. Eliminate the power valve. Use an open-element air cleaner. Maybe a more aggressive accelerator pump cam. And choose a carburetor: Holley, Edelbrock, Carter, Stromberg, Weber, Barry Grant, Autolite, or Motorcraft.

Everyone has an angle for carburetor selection and tuning. Our job is to help you make informed decisions when it comes to your carbureted fuel system.

How Do Carburetors Work?
The carburetor was invented in 1893 by Dont Bnki, a Hungarian engineer. It was a pivotal point in the development of the internal combustion engine because it revolutionized the fuel/air management process. The carburetor's function is rooted in the Bernoulli principle that air flow through the throttle bore dictates how much fuel and air enter the combustion chambers. When we open the throttle, air flow increases through the throttle bore which draws on the fuel in the fuel bowl.

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No matter how large or small, the carburetor's job is to mix air and fuel in the proper proportions to support combustion in an internal-combustion engine's chambers. How much power an engine makes is directly proportional to how the carburetor mixes the fuel and air. But the carburetor's job is only the beginning of the power process. Everything has to work together in order to yield precise fuel/air delivery. The proper combination of carburetor, intake manifold, cylinder heads, camshaft/valvetrain system, and exhaust scavenging is necessary to make the most of an engine's potential for power.

To understand how carburetors work, we have to examine each of its systems; floats, accelerator pump, idle/low-speed circuit, cruise/power circuit, and throttle bores/plates.

To successfully mix fuel and air for burning in an engine's combustion chambers, we have to get fuel into a holding chamber, atomize it, mix it with the air, and meter (a controlled supply) the mix into the engine. We don't want the engine to rev with reckless abandon. We want the ability to control the engine's power output. We do this with throttle bores and plates that control the air/fuel mixtures flow into the combustion chambers.

Floats
When fuel enters the carburetor's fuel bowl (or bowls), we want to control the amount. In normal operation, we want a smooth, steady flow of fuel into the bowl. At wide-open throttle, we want a surplus of fuel in the bowl. The float is an air pocket wrapped in either brass or synthetic foam and permeated with air. It is also a lever that bears against the needle valve. The needle valve is a fuel-flow traffic cop that prevents or allows fuel flow into the carburetor. It's typically a rubber-tipped, brass valve piston that slides inside a brass cylinder bore screwed into the carburetor body. Adjust the float level by either bending the tab that bears against the needle or moving the needle-valve seat back and forth against the float tap. Float level is generally set by the carburetor manufacturer.

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Float function differs depending on the kind of driving. When drag racing, fuel sloshes toward the back of the fuel bowl, sometimes away from where it needs to be. A carburetor fuel bowl and float that won't stop the flow of fuel is needed when we dump the clutch and haul mail. We also don't want a float and needle valve that are going to allow too much fuel into the bowl and stall the engine. This is why some carburetors are specific to drag racing.

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With road racing, the problem is fuel moving to one side of the bowl. As with drag racing, the fuel needs to stay at the jets in order to keep a steady supply of fuel for the boosters. Shelby Mustangs and Cobras, for example, had road-racing Holley carburetors with center-pivot, float-needle valves designed to stay where they belong during hard cornering, braking, and acceleration.

Accelerator Pump
Virtually every type of carburetor imaginable has an accelerator pump that goes to work as we open the throttle. As we step on the gas, the accelerator pump sprays raw, liquid fuel into the throttle bore to temporarily enrich the fuel mixture for acceleration. If we opened the throttle without using an accelerator pump, the mixture would become too lean and the engine would stall, or at the least, fall flat on its face. This short-term, fat mixture keeps combustion going as we transition from the idle circuit to the power circuit (each to be explained shortly).

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We tune accelerator pump performance by controlling how much fuel is sprayed as the throttle opens. Too much fuel can sometimes be as bad as not enough. With too much fuel, we drown out combustion and choke the guy behind us with a thick, fat-mixture fog. What's more, performance falls flat because we nearly put the fire out.

Accelerator-pump performance has to work hand-in-hand with the opening throttle and engine speed to be effective. That's why carburetor manufacturers give us the means to tune accelerator pump performance, and Holley and Barry Grant offer different accelerator pump cams (each adjustable) for tuning purposes. We can not only control how quickly the pump shot happens, we can also control how much. Besides how quickly and how much, we can also control the spray pattern in the bores with different nozzles.

Idle Circuit
Carburetors are designed with two separate circuits for idle and off-idle performance venues. At idle, we need to deliver fuel to the throttle bore a different way than when we're on the gas with the throttle open. At idle, the throttle is open just a pinch in order to keep intake-manifold vacuum high and allow air flow. We use this vacuum to draw fuel into the intake manifold via the idle circuit in the carburetor. Think of this approach to fuel delivery as sucking a soft drink through a straw. Liquid fuel doesn't burn very well, if at all. This is why we need air bleeds to help vaporize the fuel at idle. Fuel is delivered to the engine just underneath the throttle plates at idle.

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When we begin to open the throttle, two circuits come into play on the way to the power circuit. The accelerator pump provides a rich fuel spray as the throttle plates open. However, we also need a smooth transition from idle to power circuit, which comes via the off-idle ports located just above the throttle plates in the throttle bore.

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Cruising/Off-Idle Circuit
When the throttle is open and we're off the idle and off-idle circuits, fuel delivery changes considerably. With the throttle plates open, fuel is not drawn from the idle and off-idle ports but through the bores and boosters. Air flow through the boosters is what draws fuel from the bowl via the main metering jets. This works because the throttle bores are tapered mid-section, like an hour glass, to increase velocity at the boosters. With the engine running, quickly crack the throttle to wide open, and watch fuel spray from the boosters into the throttle bores. The engine will respond accordingly. The open throttle plates allow intake-manifold vacuum to draw air from the atmosphere and fuel from the bowl via the boosters. This makes the engine rev.

How we meter fuel from the bowl depends on the carburetor manufacturer and model. Holley 4150 and 4160 and Barry Grant Demon Series carburetors have two main-metering jets located in the metering block at the base of the fuel bowl. Jet size (inside diameter) determines how much fuel will flow to the boosters when the throttle is open. Edelbrock and Carter AFB/AVS carburetors have vacuum-operated main-metering jets and rods that control fuel flow when the throttle is open. The same can be said for the Rochester Quadra-Jet four-barrel carburetor found on '70-'71 429ci Fords. As we open the throttle, intake-manifold vacuum moves the tapered rods which control fuel flow. Some carburetors have mechanical fuel-metering rods that move with throttle movement, metering in more fuel.

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Power Circuit
Whereas Edelbrocks and Carters rely on a system of metering jets and rods to keep the boosters supplied with fuel in wide-open throttle conditions, Holley and Demon look to the power valve. The power valve is vacuum operated, designed to allow more fuel flow when the vacuum signal reaches a given number of inches. Holley and Demon offer a variety of power valves, numbered for identification based on the vacuum signal. We choose a power valve based on the vacuum level at which we want additional fuel. This can be a tedious process of trying different power valves until achieving the desired performance.

Cold-Starting System
Since the first person struggled with a hard-starting engine on a cold morning, the humble choke-butterfly system has helped our engines. When engines are totally cold, they need more fuel to support combustion. The choke closes off the air supply and enriches the fuel mixture for improved cold-starting performance.

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There are two basic types of choke systems, manual and automatic. Manual chokes are operated by hand via a control cable. High-performance Fords were generally equipped with manual choke systems from the factory. The rest were fitted with automatic chokes that engage when the engine is cold and disengage as the engine warms up. Automatic chokes rely on exhaust-manifold heat and a simple bimetallic coil spring to get the message that the engine is warm. Heat is drawn by manifold vacuum from the exhaust manifold or header to the choke-coil package. As the bimetallic spring expands with the heat, it gradually opens the choke and brings the throttle off the fast-idle cam.

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Proper cold-starting calls for a one-time application of throttle before starting. This causes the choke to close and the fast-idle cam to kick in. A separate throttle-stop screw jumps onto the fast-idle cam, allowing the engine to run at a fast warm-up idle. When the bimetallic coil expands and pulls the choke off, it also rotates the fast-idle cam to normal, allowing the normal-idle throttle-stop screw to close to normal at a warm idle.

Not all automatic chokes get heat from a hot exhaust manifold. Today, most aftermarket carburetors have an electric choke that gets power when the ignition is turned on. As the heating element warms the bimetallic coil, the choke begins to come off. General Motors used hot-water automatic chokes long ago, where hot coolant circulated through the choke assembly, heating the bimetallic coil for choke pull-off. Chrysler was big on choke stoves, mechanical linkages, and bimetallic coils. With V-8 engines, this kept the intake manifold decidedly hot, causing more than its share of hot-weather driving problems.

Keep It Well Fed
In our quest for mind-bending performance, you would be amazed at how many of us forget the fuel supply. We take a six-cylinder Mustang, convert it to a big-block, and wonder why the darn thing falls on its face when we dump the clutch and mash the gas. That dinky little 51/416-inch fuel line that kept your six-popper well fed won't keep a 460ci fat-block happy.

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Proper fuel-line sizing is important. As a rule and a matter of growth, size your V-8 fuel lines at a minimum of 31/48-inch diameter. For drag or road racing, you may want to step up to 71/416-inch.

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While you're at it, think about fuel pump capacity. Shop for a fuel pump and consider gpm (gallons per minute) fuel flow. If the cost bothers you, consider that a lean condition caused by inadequate fuel delivery can do major engine damage, which costs a whole lot more than a fuel pump.

Stub Stack For Performance
There are a lot of so-called performance-improving products out there with big claims they can't back up. K&N's Stub Stack isn't one of them. MCE Engines has proven in real-world dyno-testing that the Stub Stack will make a difference in performance. It smoothes the air flow into the carburetor and increases velocity through the bores. This improves low-end torque and high-end horsepower. K&N and MCE Engines aren't making any wild claims about power. Try one on your performance carburetor and see what it does for you.

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