Mark Houlahan
Tech Editor, Mustang Monthly
December 18, 2013

Photography Courtesy of the Manufacturers and Moore Good Ink

For many enthusiasts, the carburetor is some sort of black magic device that feeds an engine fuel to run. Short of bolting it on and maybe turning the base idle screw to keep the car running, few really know what's going on inside it. Out of the box, a carburetor can be close, especially if you have a mild or common engine combination. However, introduce big cams, stroker engines, large single-plane intake manifolds, and so forth, and you're going to have to tweak your carburetor's settings to not only have the carburetor perform better, but quite possibly to just get the engine to run properly. This article aims to provide you with a better understanding of the venerable carburetor, and hopefully inspire you to go ahead and perform some fine tuning yourself.

There are dozens of carburetor types to pick from and several companies that sell performance carburetors for mild-street to all-out-race combinations. We'll do our best to cover the major offerings and the most popular carburetors and how they work, but there's no denying the fact that a good book, tuning manual, or video series (like Holley's tuning DVD) is ideal for learning the complete ins and outs of your particular carburetor to get the most from it and your engine combination.

Before we delve into the major carburetor brands and design functions, we felt it best to go over some general carburetor information. Beginning with the carburetor itself, the typical unit is devised of just a few main parts; the throttle plate (also called the baseplate), the main body, and depending upon the design, there could be a separate fuel bowl or separate air horn/choke plate assembly. We'll go more into these pieces with each manufacturer in the captions.

The carburetor's three main functions are to mix air and fuel and deliver the combustible mixture to the engine, control the engine's speed, and adjust the air/fuel ratio. Fuel is delivered to the carburetor via the fuel pump, either mechanical or electric, and stored in the carburetor's fuel bowl(s). Airflow through the carburetor is created by vacuum from the engine's pistons' downward stroke, which draws air through the carb, intake, and finally into the combustion chambers. The amount of fuel drawn into this airflow is controlled by the carburetor's venturi and choke plate. The air speed increases through the venturi, creating a pressure drop that allows the fuel to be pulled into the air stream from the carburetor and ultimately into the engine. Since atmospheric pressure is a critical player in carburetor function and tuning, it is noteworthy to remember that what works for a carburetor at sea level will not be the same for a carburetor employed at 4,000 feet above sea level.

Seeing the calculated cfm is 767, a 750-cfm four-barrel carburetor would seem the logical choice. One further thing to consider is that the above calculation does not adjust for volumetric efficiency (VE) into consideration. An engine's VE rarely achieves 100 percent unless it is supercharged or turbocharged. A typical production engine will have a VE of around 70-75 percent, while a performance engine's VE will be approximately 10 percent higher. If we recalculate the cfm requirements with VE in mind, we'll get a truer requirement for our carburetion. So, if we consider our 408ci Windsor a “performance” engine with 85 percent VE, the actual cubic inches of airflow is now 347 cubic inches. If we perform the above calculations again with our new value, it will look like this: 347 (cid) x 6,500 (rpm) / 3,456 = 654 cfm

So you can see that our paper tiger 408ci Windsor would be very happy with a 650-cfm four-barrel instead, and the 750-cfm unit would be over-carbureting the engine for most applications short of high-rpm racing use. You can see these numbers in real world applications if you consider the original carburetor requirements of some of Ford's factory V-8 engines. For a similar example, the 428ci FE used a 600-cfm of carburetor and many 289ci small-blocks used just a 480-cfm unit.Even though we've done the basic airflow calculations, there are other things to consider, such as cam lift and duration, axle gearing, manual or auto trans, vehicle weight, and more. Carefully consider your engine's airflow needs with these factors in mind, and speak with the tech lines of the carburetor companies. One of the most common carburetor issues they all see is too large of a carburetor being used for the application.

Once you've determined the cfm of your carburetor, there are several other functions/features to consider. Arguably the most important feature after cfm is whether you need/want a vacuum-secondary or mechanical-secondary-based carburetor. In a nutshell, mechanical secondaries are controlled by a mechanical connection to the primary throttle bore linkage, whereas the vacuum-controlled secondary opens the secondary throttle bore via a vacuum signal related to engine load. Both styles can be tuned/adjusted to fit a vehicle's weight, operating range, and so forth, though many camps agree a vacuum-secondary carb is preferred for street use and/or with an automatic transmission. It's common to see a mechanical-secondary carb in the wrong application giving the operator poor fuel economy, hesitation on hard acceleration, and unacceptable driveability on the street. Further considerations need to be made before purchase such as linkage type, electric or manual choke, fuel inlet design, and more. Once again, we strongly recommend contacting a manufacturer's tech line for suggestions.

Choosing the proper carburetor for your application should be your first consideration, as you'll find many carburetors with similar features (vacuum secondaries, electric choke, and more) in numerous cfm (cubic feet per minute) offerings. For example, you'll find Holley 4150 four-barrel carburetors available from 390-cfm to 1,000-cfm, depending upon model sub-series. What's the best fit for that 408ci 351W stroker you just built? It's probably safe to say the 390-cfm unit is too small and the 1,000-cfm unit is too big, but what's the ideal carburetor sizing for your new engine? Is it 600-cfm, or perhaps 750-cfm? Here's a bit of basic math to get you in the ballpark.

Too small or too large of a carburetor is something you simply can't “tune around,” so to determine the cfm requirement of your engine, take the cubic inch displacement and multiply it by the maximum rpm of the engine, and then divide by 3,456. The calculation will look like this example for our hypothetical 408ci Windsor:408 (cid) x 6,500 (rpm) / 3,456 = 767 cfm

Demon Carburetors

When Demon carburetors first came on the market, they were offered in several different performance levels from street to race, with varying feature sets applicable to the intended market. A few years ago the company filed for bankruptcy protection and the performance aftermarket looked like it was going to lose a viable source for performance carburetion. Thankfully, new ownership picked up the pieces and not only does Demon offer the original 4150 Holley-style modular Road, Speed, and Mighty Demon carburetors, but it is making waves with a new Street Demon carburetor. The Demon line has been known for its billet metering blocks and baseplate, Idle-Eze feature (adjusting the base idle via a screw in the air cleaner stud socket that allows idle changes without moving the primary throttle plates), and quick-change vacuum secondary pod.

Demons feature four idle mixture screws, one on each side of the primary and secondary metering blocks

Some Demon models feature a choke horn but do not come with a choke plate (see mechanical secondary model in previous photo). If your application calls for a choke to air in cold starting, Demon offers an electric-choke conversion kit that the end user can easily install.
On Demon’s vacuum secondary offerings, the quick-change diaphragm spring design allows for tailoring the opening rate of the secondary throttle shaft with the simple change of a spring.
The Demon’s metering block is made from billet aluminum and houses the jets, power valve, idle mixture screw, and more. The metering block is where the carburetor’s fuel is metered and mixed with air. You can see the various components in this call-out diagram. A) Idle wells B) Main wells C) Main well exit D) Idle-mixture adjustment screw E1) Transfer slot discharge E2) Idle mixture channel controlled by adjustment screw F) Emulsion bleeds G) Power valve
On the opposite side of the metering block you’ll find the main jets. The main jets of the carburetor are a fuel-control orifice and are numbered by size. Changing the jets will affect wide-open-throttle power. Jets are available in pairs or you can purchase a tuning kit with several sizes; perfect for dyno or track tuning.
This shot of the power valve removed from the metering block shows its construction, which includes a diaphragm, check valve, and spring. The power valve gives the carburetor additional fuel in low-vacuum conditions (such as full throttle load). You will find power valves available in different vacuum ratings to aid in tuning the carburetor further.
The Demon’s accelerator pump is found on the primary fuel bowl only, unless it is a mechanical-secondary carb, which has one on the secondary fuel bowl as well. The accelerator pump gives the carburetor a “shot” of fuel on initial acceleration to prevent stumble/lean condition. The accelerator pump can be adjusted via the pump arm adjustment screw and additional tuning is made possible with larger pump capacities and different nozzle orifices.
Mentioned earlier in our opening paragraph on the Demon line, the Idle-Eze function allows base idle speed changes to be made to the carburetor without disturbing the primary throttle blade angle, potentially disturbing the carburetor’s vacuum signal for the transfer slots.
Demons feature four idle mixture screws; one on each side of the primary and secondary metering blocks. Adjusting the idle mixture screws takes a light touch (often just a 1⁄8 to ¼ turn) and should be performed with a vacuum gauge. You want the highest vacuum reading possible, moving from one screw to the next.
All Road, Speed, and Mighty Demon carburetors feature center-hung floats. Ensuring dry float measurements are correct using a float gauge or specific drill size. On-car adjustments can be made using the float adjustment nut on each fuel bowl.
Demon’s newest carburetor is its Street Demon and it takes cues from the original Carter AFB design. The one-piece main body includes a fuel bowl with the top cover gasket above the fuel line to virtually eliminate leaks. The carburetor is available in 625- and 750-cfm models in burnished or hand-polished finish, and with an optional polymer main body. Like other Carter-based carbs, it features a dual bolt pattern to work on square- and spread-bore intake manifolds.
Unlike the traditional “four-barrel” carburetor, which features four individual throttle bores, the newest Demon features a single secondary plate called the Goggle Valve. The Goggle Valve is mechanically connected to the primary throttle valves via a progressive linkage that starts opening at 30-degrees of throttle. The air valve located over the Goggle Valve (seen in the previous picture looking like a choke plate) is vacuum operated like a vacuum-secondary throttle in a modular carburetor, and it is easily tuned via a spring tension screw on the side of the carburetor.
Demon Carburetion offers these handy tuning kits that include metering rods, step-up springs, and primary and secondary metering jet sizes. The metering rods and step-up springs are easily accessed from the top of the carburetor for tuning purposes, while the Street Demon’s jets are accessed by disassembling the carb. ▶
Unlike a traditional modular carb, the new Street Demon uses metering rods and step-up pistons with springs for fuel enrichment instead of a power valve. The metering rods move via vacuum signal and restrict flow through the carburetor’s jets.

Edelbrock Carburetors

As the king of aftermarket intake manifold design and production, Edelbrock certainly knows a thing or two about airflow and an engine's fuel demands. Instead of designing a new carburetor from scratch Edelbrock engineers started with a known good product and simply made it better. If the Edelbrock Performer and Thunder series carburetors look familiar to you, then most likely you've messed with a pal's Carter AFB or AVS carburetor back in the muscle car heyday. Today, Edelbrock has refined the original Carter design and offers 16 Performer versions from 500 to 800 cfm with manual or electric choke, and available in satin, black powdercoat, EnduraShine, and vintage look. The Thunder series comes in 18 part numbers from 500 to 800 cfm in manual and electric choke, with satin and EnduraShine finishes.

Moving up to Edelbrock’s Thunder series, which utilizes the later Carter AVS design, nets you an upgrade to the adjustable air valve secondary plate (similar to the new Street Demon) that is adjustable on the carb with no extra parts required. The Thunder series also features secondary booster venturi clusters.
In this cut-a-way you can see the accelerator pump circuit of the Edelbrock carbs. Unlike a modular carb design that uses a square diaphragm and pivoting lever, the Edelbrock design uses a piston with a spring that is actuated via external linkage. This linkage can be adjusted (bent) or positioned in different actuation lever positions. Just be aware that the accelerator pump cavity shares the fuel level with the main body, so if the accelerator pump is raised higher than the fuel level, you’ll have trapped air to compress before you get your “pump shot.” As such, overall float level is critical to accelerator pump function.
Another way to improve the accelerator pump’s function is to change the discharge nozzle size. With a single twist of a screw you can change out the factory nozzle to one of the three in Edelbrock’s #1475 accelerator pump nozzle kit. If your Edelbrock has a stumble on acceleration, often the discharge nozzle size is usually the issue.
While the Demon uses a three-step metering rod design, the Edelbrock carburetors use the traditional two-step metering rod. The metering rods might look similar, but they should not be swapped between the different companies’ offerings.
Like all carburetor manufacturers, Edelbrock offers a slew of tuning, service, and rebuild parts. For the typical Edelbrock carburetor, the calibration kit for the model carb you’re tuning is the best place to start.

Holley Carburetors

Holley's four-barrel carburetors have been a staple of the aftermarket performance scene for half a century and have been used by Ford for OE carburetion solutions since 1957. It's no wonder that the company's modular design has been the basis for a whole cottage industry of upgrade parts and complete custom carburetors for motorsports solutions in everything from off-road buggies to oval track racing. Lift the hood of a classic car at a cruise night or test-n-tune at the track and your chances of finding a Holley four-barrel sitting on the intake manifold are better than average. Over the years, the Holley design has changed little, but constant improvements keep the Holley 4150/4160 line of carburetors at the forefront.

Moving to the 4160-based Holley, you’ll notice a few differences. For 4160 Holleys up to 600 cfm, the carburetor comes with side-hung floats and a single fuel feed with an external transfer tube for the rear fuel bowl on the driver side. You’ll also note that there is no visible metering block at the rear of the carburetor.
Moving to the 4160-based Holley, you’ll notice a few differences. For 4160 Holleys up to 600 cfm, the carburetor comes with side-hung floats and a single fuel feed with an external transfer tube for the rear fuel bowl on the driver side. You’ll also note that there is no visible metering block at the rear of the carburetor.
By comparison, this is a 4150-based Holley Double Pumper—a mechanical secondary carb—in Holley’s popular tumble polished finish. Note the lack of vacuum diaphragm housing. Also visible is a manual choke cable bracket and “four-corner idle circuit” with idle screws in the rear metering block.
When you use a 4160 Holley over 650 cfm, the carb will come with the more traditional looking center-hung fuel bowls and dual-feed fuel inlet on the passenger side of the carb.
While the 4150 Holley uses a secondary metering block as well (with or without the power valve), the 4160 model uses a metering plate. The plate is a cast plate with specific sized holes for fuel metering. While it is common for Joe Shadetree to drill these out, you can only go larger. The better bet is to order the properly sized metering plate from Holley. The rear fuel bowl completely covers the metering plate, and as such, needs to be removed for access to the metering plate for service/tuning as well.
All Holley modular carburetors use a metering block for the primary venturi side of the carburetor. These blocks hold the main jets and a power valve. Accessing the metering block for jet changes or to replace the power valve entails draining the fuel from the fuel bowl and removing the fuel bowl for access.
The Holley’s accelerator pump system differs from others in that instead of bending a linkage or changing the linkage mounting hole position, Holley uses plastic cams on the throttle arm. Holley’s accelerator pump cams come in a nine-piece kit with each cam offering two mounting positions. That’s 18 different accelerator pump profiles to choose from. Of course, the accelerator pump discharge nozzles are available in numerous diameters for further tuning of the accelerator pump shot as well.
Some of Holley’s newer four-barrel offerings, such as its Street Avenger aluminum four-barrel, include Holley’s quick-change vacuum secondary spring kit as standard equipment. If your Holley does not have this upgrade, you can purchase the kit separately. And it makes easy work of tuning the secondary throttle opening on vacuum secondary carburetors.
Holley offers replacement jets in numbered pairs, 10-packs, and this huge tuning assortment. Holley jet numbers range from 40 to 110, with inside diameter increasing as the jet number increases. Tuning the jet sizing can be accomplished with road/track testing or a dyno (engine or chassis).

Required Reading

As you might suspect, we're just touching the tip of the carburetor tuning iceberg here. The numerous carburetor part numbers, features, and options would take every page in this magazine to thoroughly discuss. As such, we can't recommend the proper educational material enough. Starting with tuning/rebuilding manuals, either from aftermarket publishers or from the carburetor companies themselves for an in-depth look at how carburetors work in general, as well as the specific functions of your model carb. You'll also find helpful exploded views/cutaways, FAQs, and tuning videos on each manufacturer's website as well. Many carburetors today come with an installation and tuning DVD right in the box—don't chuck the disc in your pile of paperwork on your workbench—pop it into your computer or DVD player and watch it.

Keep it Clean
A clean fuel system is paramount to proper function of your carburetor. From the moment the fuel is pumped into your vehicle, there are numerous chances for it to pick up dirt and debris that will foul your carburetors intricate fuel and air passageways. This can lead to a poor running, stumbling, stalling vehicle that will have you cursing the gods of carburetion. Keep these simple basic facts in mind and your carburetor (and fuel system) will provide you trouble free operation.

Always run an air cleaner
Street cars aren't race cars running for nine seconds at a time two or three times on a weekend! Having an air filter in place ensures the air entering the carburetor (and thusly the engine) is clean of dirt and contaminants that can foul a carburetor (and ruin the internals of an engine over the long term).

Always run a fuel filter
A quality fuel filter should be used between the fuel pump and carburetor. Do not use cheap plastic filters and if you have a glass filter for easy viewing, get rid of it now. Glass does not belong in a fuel system. A high-flow aluminum filter with replaceable or cleanable filter media is best.

Keep your air bleeds clean
Dirt and crud can find its way to your carburetor's air bleeds in the air/choke horn area. Keeping them clean ensures the carburetor gets the air it needs through the right passages for the carburetor to work. A simple blast with carb cleaner to clean away dirt is all you need. Do not run a wire tool through the air bleed to clean it; you will damage the air bleed.