Ford Engine Carburetors - Carburetors Explained
Getting Power And Making The Most Of The Fuel/Air Charge Begins Here
From the December, 2006 issue of Modified Mustangs & Fords
By Jim Smart
Photography by Jim Smart
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.
In over 100 years, carburetor...
In over 100 years, carburetor basics haven't changed much, and the principle remains the same as in 1893 when the carburetor was invented. It is all about air flow, speed through the bores, and how each draws fuel from the fuel bowl. This is a Holley 4160 carburetor provided by MCE Engines of Los Angeles.
There has to be enough fuel...
There has to be enough fuel in the fuel bowl to keep the engine supplied. The float and needle valve control how much fuel enters the carburetor. As the float falls with the consumption of fuel, it relieves pressure on the needle valve, allowing more fuel into the bowl. The float rises against the needle valve, closing off the flow of fuel to the bowl. In normal operation, float movement isn't even perceptible. It maintains a smooth flow of fuel into the bowl.
This is a center-pivot Holley...
This is a center-pivot Holley fuel bowl with a brass float. Brass floats are sometimes recommended because foam floats can take on fuel, sink, and stop doing their jobs. Brass floats can also leak when a solder seam gives out and the float fills with fuel. Each type has its advantages, and every carb tuner has a preference.
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.
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.
These are foam floats for...
These are foam floats for Holley center-pivot carburetors. The float shape determines how it performs and what it is designed for. The shape determines how sloshing fuel effects float operation. Road-racing floats are shaped differently than drag-racing or street floats. In any case, we want the float to remain stable at all times to ensure fuel remains around the jets.
This baffle on a Holley ensures...
This baffle on a Holley ensures fuel doesn't slosh out of the bowl under extreme driving conditions. It's always a good idea to have these in your Holley.
These are two examples of...
These are two examples of float needle-valve assemblies for Holley carburetors. We like this arrangement because the float-needle valve remains stable under all kinds of driving conditions. These are also the easiest float-needle valves to adjust in the industry.
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.
The Barry Grant Demon fuel...
The Barry Grant Demon fuel bowl bears a resemblance to the Holley counterpart: a center-pivot design that provides stability under a variety of conditions.
Edelbrock and Carter float...
Edelbrock and Carter float systems look like this, with two separate floats on the left and right. Primaries and secondaries feed off the same bowls. This is a good design because the floats remain stable under a variety of conditions.
This is a generic accelerator-pump...
This is a generic accelerator-pump circuit to give you an idea of how it works. When we lift on the throttle, fuel enters the accelerator-pump cavity via a check valve that allows fuel to flow one way but not the other. When we step on the gas, the check valve closes, allowing one-way fuel flow to the nozzles in the throttle bores.
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.
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).
These are accelerator pump...
These are accelerator pump nozzles in an Edelbrock carburetor. Note that each directs liquid fuel into the venturi (throttle bore) as the throttle plates are opened. Nozzles vary in size and length depending on the need.
With Holleys, Autolite 2100/4100s,...
With Holleys, Autolite 2100/4100s, and Barry Grants, accelerator pump tuning is also controlled by pump-cam selection, positioning, and pump-cavity size.
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.
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.
Another type of accelerator...
Another type of accelerator pump found in Edelbrock, Carter, and Rochester carburetors is this spring-loaded design that allows the flow of fuel through the nozzles at a predetermined rate. No matter how much you mash the gas, this design allows only a predetermined amount to flow.
This is the idle circuit in...
This is the idle circuit in a typical carburetor design. In its most basic form, the idle circuit needs help to allow an engine to idle. At idle, fuel is drawn from the fuel bowl up to the top of the carburetor as shown. The mixture screw at the throttle plate controls how much fuel flows through the idle circuit.
As we open the throttle, the...
As we open the throttle, the air flow past the throttle plates changes. There are two idle ports, one below the throttle plate and one above. At idle, we are on the idle port below the throttle plate. As we come off idle, we transition from the idle port below the throttle plate and draw fuel from the idle port above. This keeps the fire lit until we move into the power circuit.
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.
As the throttle opens, we...
As the throttle opens, we cruise from the bottom idle port to the idle port above the throttle plate. The more we open the throttle, we're handing fuel delivery off to the power circuit, where fuel enters the throttle bores (also called venturis) via the boosters, getting the fuel into the mainstream-bore air flow.
Here's another example of...
Here's another example of the idle circuit in an Autolite 2100 series two-barrel carburetor. Fuel is drawn from the fuel bowl to the idle port below the throttle plates. As we open the throttle, fuel delivery transitions from the bottom idle port to the off-idle port above the throttle plate. Some carburetor throttle bores are slotted where the plate closes. This allows a smooth transition from idle to power circuit.
The idle-transfer slot on...
The idle-transfer slot on a Holley 4160 handles fuel delivery to the power circuit. Think of it as the off-idle port above the throttle plate.
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.
When we adjust the idle mixture,...
When we adjust the idle mixture, this is exactly what we are doing. These needle valves adjust the amount of fuel flowing to the idle ports at the throttle plates. When we close them, we cut off fuel flow. When we open them, we increase fuel flow. The idle-mixture adjustment controls the air/fuel ratio we are getting at idle speed. It has no bearing on mixture when the throttles are open. A weak spot of Holleys is the metering-block passages for the idle circuit. When they get clogged with dirt, idle quality suffers. What's more, it doesn't change when we turn the mixture screws in.
This is a basic example of...
This is a basic example of the cruising or off-idle circuit. With the throttle open, vacuum is created through the throttle bore or venturi and draws fuel through the booster in each bore into the slipstream. The fuel is atomized (vaporized) and flows past the throttle plates into the intake manifold.
In this cross-section of a...
In this cross-section of a carburetor, note the booster. As its name implies, the booster helps accelerate the flow of air through the bore, giving the fuel particles velocity. It also improves the fuel-spray pattern.
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.
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.
We tend to think suction (manifold...
We tend to think suction (manifold vacuum) is the reason fuel comes from the bowl via the boosters, but it's actually the atmospheric pressure difference between the fuel bowl and throttle bore that draws the fuel through the boosters. Whenever we have less air pressure in the throttle bores than we do in the fuel bowl, fuel will flow toward the low-pressure area.
Here is a close-up illustration...
Here is a close-up illustration of the main fuel nozzle, also known as a main discharge tube. Note the air bleeds which help atomize the fuel.
This close-up of a Holley...
This close-up of a Holley 4160 shows the air-bleed holes for improved fuel atomization. They are factory-sized on most Holley and Demon carburetors, but some newer models have adjustable air bleeds that can be changed as necessary for finite carburetor tuning. Note the accelerator-pump discharge nozzle between the primary bores. Also note the fuel-discharge boosters that get fuel from the main-metering jets in the fuel bowl.
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.
Edelbrock, Carter AFB/AVS,...
Edelbrock, Carter AFB/AVS, and Rochester Quadra-Jet carburetors have a different fuel-metering system design than Holley, Autolite, and Demon. Fuel metering rods are either vacuum or mechanically operated. These rods are tapered and hang in the middle of the main metering jet to control fuel flow through the jets. We can change jet and metering-rod sizes depending on performance demands.
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.
These are boosters in a Barry...
These are boosters in a Barry Grant Demon carburetor. Boosters help draw fuel from the bowl via the main metering jets. The shape of the booster both accelerates air flow through the booster and improves the fuel-spray fan pattern, which improves fuel atomization. Adjustable, replaceable air bleeds improve performance even more. The Demon carburetor is revolutionary to performance buffs because it takes carb tuning to a new level.
The Edelbrock and Carter AFB/AVS...
The Edelbrock and Carter AFB/AVS have a cruise circuit like this. These boosters can be swapped as needs dictate and are easy to service.
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.
The Holley 4150/4160 main...
The Holley 4150/4160 main metering system includes this metering block, which includes two main metering jets and a power valve located mid-block. Jet sizes can be varied to fit your engine's needs. Fuel is drawn through these jets to the boosters. The larger the jets, the more fuel is drawn from the bowl.
When driving becomes extreme,...
When driving becomes extreme, we want to keep the boosters supplied with fuel. Jets with extensions help keep fuel going to the boosters in hard braking and cornering.
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.
This is one of four main metering...
This is one of four main metering jets in an Edelbrock/Carter carburetor. A vacuum-operated tapered rod (not pictured) fits into the jet to control fuel flow to the boosters. These carburetors have two jets for the primaries and two jets for the secondaries.
When we speak of primaries...
When we speak of primaries and secondaries, we're talking throttle bores, barrels in shop slang. The two bores on the left are primary throttle bores, which are used in normal driving. On the right are two secondary bores which are opened mechanically or with vacuum when we mash the gas for more power.
This is the view from above...
This is the view from above of a four-barrel carburetor with four throttle bores and four boosters. Again, we cruise on two barrels in normal driving and four barrels when the throttle is wide open.
Holley and Demon carburetors...
Holley and Demon carburetors have the power valve screwed into the metering block as shown.
Here's a typical Holley, Demon,...
Here's a typical Holley, Demon, Autolite, Motorcraft automatic choke. Note the bimetallic coil inside the black, plastic cap. The fast-idle cam is a nylon part just below the choke mounting screw. Lean means bringing the choke off sooner. Loosen the three screws that retain the cap and slightly open the throttle to adjust the choke. You want the choke slightly closed. Be sure to adjust it with the engine cold, then check performance. If it is belching a filthy, rich mixture at the tailpipes, lean it out a little by opening the choke some more.
This is an Edelbrock carburetor...
This is an Edelbrock carburetor choke in the cold-start position, partially closed to enrich the mixture.