Easily accessible dual float bowls, power valves, and jets make the modular Holley-style carburetor one of the easiest to tune. This unit is a Barry Grant Mighty Demon. The dual accelerator pumps, in conjunction with mechanical secondaries, allow transition from part- to full-throttle without bogging. Few carbs have the street cred of the Holley-style double-pumper.
Vacuum vs. Mechanical
A carburetor's secondary throttle blades open up to provide additional airflow under heavy acceleration. This can be accomplished mechanically or via vacuum assist, and each has its pros and cons. For heavy vehicles with tall gears, it's generally accepted that a vacuumsecondary carburetor provides superior streetability and gas mileage. This is because the secondaries open up gradually as engine vacuum in the primary venturis increases with rpm. Mechanical secondaries, on the other hand, are directly linked to the gas pedal. Typically, mechanical secondaries will begin to open at 40 to 45 percent throttle. They have a reputation for "hitting harder" than a vacuum-secondary carb at the expense of driveability and are better suited for more serious engine combinations. Additionally, some (but not all) mechanical-secondary carbs feature a second accelerator pump, so some (but not all) mechanical-secondary carbs are double-pumpers. Despite their reputations, vacuum-secondary carbs can support massive horsepower, and many enthusiasts flog mechanicalsecondary carbs with great success on the street. The introduction of Barry Grant's vacuum secondary King Demon (4500 series) recently is a good example of this.
A trick feature of the Mighty Demon is the annular discharge booster. The setup yields a finer mist of fuel for improved atomization and throttle response. Boosters are easily replaced if altering the signal strength is required. The greater the booster's restriction to flow, the stronger the carb signal-and vice-versa.
Tuning
A detailed guide to carb tuning is beyond the scope of this story. Dozens of books have been written on the topic, but the question is, is it even worth bothering with? "Carb manufacturers have done an incredible job of engineering their products right out of the box," Judson explains. "All these bracket racers and street squirrels want to tinker with them, but they mess them up more than they ever improve them. As long as you size carbs properly, they're so good out of the box, it's unreal. Even if you're a great carb tuner, you might only get another 1 percent out of a motor."
Nonetheless, there is a time and place for altering the factory's calibrations. "Out-of-the-box carbs are fine for street/strip motors, but if you're building a race motor, you need a race carb," says Dave Braswell of Braswell Carburetion. "Race cars operate in environments that street cars never see. For instance, since drag cars launch so hard, we've developed carbs that can deliver fuel at 3 g's without uncovering the main jets."
Granted, that's an extreme scenario, but where most gains can be picked up is in the fuel curve. Production carburetors are intentionally tuned to run slightly rich at high rpm for safety. Flattening out the curve is best left to a pro, but doing so can yield dividends. "Every engine wants a different fuel curve, and the fuel port passages in a carb are like the cross-sectional area of the ports in a cylinder head," explains Patrick James of ProSystems Carburetors. "We modify the fuel passages to make the emulsion process more active and custom-tailor the diameter of the fuel ports and fuel curve to each application. There's more to it than simply adding more fuel; it must be introduced in a burnable fashion."
The Holley-style modular design offers easy fuel level-adjustment, and it's accomplished similarly to this Demon carb cutaway. The needle and seat can be externally adjusted by simply turning this screw.
Holley Carbs
Numbers aren't always an indicator of quality, but the Holley modular fourbarrel is the most popular performance carburetor in the world. George and Earl Holley started building carburetors in 1904, and the company has since produced over 100 million units. After introducing a pair of 370-cfm fourbarrels in the early-'50s-Models 2140 and 4000-Holley launched its legendary 4150 model in 1957. Amazingly, that same basic design architecture has been chugging away for over 50 years, with continual refinements increasing airflow from 400 cfm in 1957 to more than 1,000 cfm today.
The 4150's big brother-the model 4500 Dominator-was developed by Holley for Ford in the late-'60s to assist the company in its quest for NASCAR and Trans Am racing glory. The original Dominator flowed 1,150 cfm and was used on Ford's 429- and 302ci race motors. Today, the Dominator is available in flow ratings ranging from 750 to 1,050 cfm and is the carburetor of choice for hoards of hardcore racers.
Located in the metering block, the power valve is easily accessible and replaceable. Power valves are rated at the inches of manifold vacuum at which they open. For instance, if an engine pulls 7.5 inches at idle, the carb should be fitted with a numerically lower (2.5-,3.5-, 4.5-, 5.0-, or 6.5) power valve to prevent running overly rich.
In response to the changing needs of the OEs due to smog concerns, Holley has introduced countless carburetor models over the years. These include emission-friendly models, and even carbs designed as high-performance Quadrajet replacements. Nevertheless, the 4150 and 4500 series carbs are unrivaled in their popularity and street cred. Over the years, various manufacturers (such as Barry Grant, Brasswell, and QFT) have introduced their own product lines based roughly on the Holley modular architecture.
Quadrajet Carbs
Vilified by the masses yet embraced by experienced carburetor tuners, the Rochester Quadrajet has gotten a bad wrap over the years. Dubbed the "Quadrajunk" by the uninitiated due to its complexity, the Q-jet is quite possibly the most versatile and advanced carb ever built. "Since it was designed as a GM production carb when emissions laws were getting stricter, it had to, be very accurate," says Sean Murphy of Sean Murphy Induction. "When tuned properly, a Q-jet is hard to beat. Fuel mileage, top-end power, and low-end torque-it can do it all."
As with EFI, it's the Q-jet's precision that can make it finicky at times. Although Holleys and Carters can be extremely forgiving of tuning errors, this isn't the case with the Q-jet. "Due to its precision and accuracy, Q-jets won't let you be nearly as sloppy with tuning compared to the other carburetor platforms out there. You can't take a Q-jet that was running well on one motor and stick it on another motor and expect it to run right. Even small changes in compression or cam can really upset the tuning."
Large throttle angles push the accelerator pump's lever against its diaphragm to initiate fuel enrichment when throttle angle increases. The pump can be tuned with interchangeable pump cams to alter its fuel curve.
Built by the Rochester Products division of GM, the Quadrajet first appeared in 1965. Designated the 4M, it featured small-bore primaries for improved throttle response and fuel economy and large-bore secondaries to meet fueling demands under heavy acceleration. During its reign of over 20 years, the venerable Quadrajet was used by every GM division. It survived well into the '80s, even implementing computercontrolled engine management over many of its functions, until finally being phased out in favor of EFI.
As a production piece, the Q-jet was never designed to be a highperformance carb. However, all Q-jets flow a minimum of 750 cfm, and some flowed a very respectable 800 cfm. Furthermore, NHRA Stock & Super Stock racers have been pushing well over 600 hp with Q-jets for many years. The Q-jet was continually refined over the years, and the first major redesign came in 1975. Called the M4M, o rmodified 4M, it boasted a larger fuel bowl capacity, a revised adjustable part-throttle (APT) system, and larger primary bores. The Q-jet went relatively unchanged until the middle of the '80 model year, when it implemented electronic controls and was dubbed the E4M (Electronic 4M). Consequently, the '75-'80 castings are the most coveted by hot rodders.
Idle screws located on the metering blocks can often cure a rough-idling motor. Turning the screw clockwise leans the mixture, while turning it counterclockwise richens the mixture. However, in certain emission "reverseidle" carbs, the exact opposite is the case.
Carter Carbs
Noted for its simplicity, reliability, and adaptability to a myriad of applications, the original Will Carter fourbarrel (WCFB) carburetor dates back to 1952 when it was first introduced on Buick straight-eight engines. It was a cutting-edge piece for its time, used as factory equipment on C1 Corvettes and Chrysler Hemis, but increasing power levels of the era demanded higher air- flow capacities. To meet these needs, the Carter AFB (Aluminum Four-Barrel) was introduced in 1957 and was used as an OE carb by Chrysler, Ford, and GM throughout the years. The AFB earned a reputation for potent off-the-line punch, and engines such as the Pontiac 421 Super Duty-which used a pair of AFBs-helped reinforce that image. Although Carter didn't rate the flow of its carbs, the AFB is believed to have flowed between 450 to 625 cfm.
By 1966, the AFB was superseded by the AVS (Air Valve Secondary) carb and was used primarily by Chrysler. The AFB and AVS look almost identical from the outside, which is partly why the AVS never gained widespread acceptance by hot rodders. "The biggest difference between the AFB and the AVS is the AVS has a spring-loaded secondary air valve as opposed to the AFB's counterweighted air valve," explains Smitty Smith of Edelbrock. "The AVS' air valve is adjustable to better suit heavy vehicles, whereas the AFB's valve is not adjustable." Another key difference is the AVS' lack of secondary booster venturis, although Edelbrock has revised the original design by adding boosters to its Thunder Series AVS carburetors.
Long after the AFB had been phased out, Carter brought it back in the mid- '70s as the Model 9000, updated with an electric choke, emissions provisions, and OE throttle-linkage compatibility. Due to the rise of factory EFI in the mid-'80s, Carter lost significant market share and was sold several times before being acquired by Edelbrock. Today, the company offers AFB and AVS carbs, with flow ratings up to 800 cfm, with both square-flange and spread-bore bolt patterns.
On a modular Holley architecture, the idle and high-speed air bleeds are located next to the boosters. Larger bleeds lean the air/ fuel mixture and delay fuel delivery, while smaller bleeds richen the mixture and initiate fuel flow earlier. Carb manufacturers advise against altering air bleeds since tuning with smaller or larger jets is much safer and more forgiving. | 
Although it guarantees a fuel spill, simply removing the fuel bowls exposes the metering block and jets. This access makes tuning in the pits a painless affair. Fuel spillage can be avoided with a small spill can made by Moroso or Barry Grant .(An aluminum soda can cut in two will also do the trick on a short intake manifold.) | 
With an increasing number of hot rodders who want to keep their cars all-original, the Quadrajet's popularity has recently surged. Rebuilt using '76-'80 Chevy castings that feature side fuel inlets, these Sean Murphy Induction units are a work of art and listed for just over $300. |
The most distinguishing characteristic of the Quadrajet is its odd-looking spreadbore baseplate. It is this extreme disparity in size between the Q-jet's primaries and secondaries that give it its legendary blend of throttle response, fuel mileage, and top-end power. | 
All Q-jets feature mechanically actuated secondaries. Like the Carter AVS, an air valve covers the secondary bores and gradually opens as air speed increases. The rate at which the valve opens can be adjusted by changing the spring tension. | 
The needle-and-seat assembly is buried beneath the air horn. One of the Q-jet's drawbacks is its small fuel bowl capacity. On a healthy motor, sucking the bowls dry under prolonged WOT operation is a concern, especially with a fuel system that isn't up to par. |
These internal passages are notorious for leaking, so SMI plugs them shut with epoxy. | 
Due to the urging of the EPA, GM closed over the idle mixture screws in the baseplate late in the Q-jet's production run. Hot rodders and mechanics simply chiseled off the casting material and adjusted it away. | 
Positioned between the primary bores, the Q-jet's power piston controls the height of the tapered metering rods in the jets, therefore increasing or decreasing fuel flow. The piston rides on a spring and is pushed shut by engine vacuum at idle. |
Situated in front of the primaries, the Q-jet's accelerator pump operates by forcing fuel trapped beneath it through internal passages in the air horn. Fuel is then routed to holes in the main bores of the primaries that are located next to the boosters. | 
A Q-jet's fuel jets are integrated into the baseplate itself at the bottom of the fuel bowl. The primary jets are easily removed, but the secondary jets (which, are actually fixed-diameter discs) are not. Consequently, fuel flow in the primaries can be altered by changing out the jets or metering rods. | 
The entire secondary enrichment system on a Q-jet is integrated into the air horn. Surrounded by emulsion tubes on both sides, the two center metering rods control the rate of fuel flow into the secondary bores. The metering rods are raised by a hanger attached to the secondary air valve flap when throttle angle increases. |
The triple-venturi area of a Q-jet creates three distinct areas that provide a particularlyb strong carb signal to the main fuel system. This affords extremely precise fuel metering. While the outer rings can easily be removed to increase airflow on the primary side, the benefits of doing so on a typical street/strip motor are questionable. | 
Now manufactured by Edelbrock, modern Carter-style carburetors are a popular choice on many street machines. Edelbrock offers both AFB and AVS units that flow up to 800 cfm. Redesigned boosters and a variety of linkage adapters are among several changes the company's engineers have made to the original Carter platform. | 
Carters (now Edelbrocks) feature internal fuel bowls located on the left and right side of the carb assembly, with a channel that equalizes fuel level on both sides. Located in the bottomright is the accelerator pump, which directs fuel to the discharge nozzles (positioned between the primary venturis) when manifold vacuum drops abruptly. |
To achieve fuel enrichment, Carter-type carbs employ a combination of jets and metering rods on the primary side, and jets only on the secondary. Like the Q-jet, a Carter's jets are located on the float bowl floors and are removable with a wide-blade screwdriver. | 
In lieu of a power valve, Carter-type carbs use tapered metering rods that are attached to spring-loaded pistons for each primary venturi. They move up and down in concert with manifold vacuum, and varying spring rates and rod diameters alter the air/fuel ratio. The piston assembly is easily accessible by removing its retaining plate on the air horn. | 
The Carter's idle-mixture screws are conveniently located on the front of the carb.Factory calibrations are usually very close to ideal, and turning the screws as little as a 1/8-turn will affect idle quality. It's usually good practice to set the left and right screws as close to each other as possible. |
On Carter-style carbs, float level is determined by flipping the air horn upsidedown and measuring the distance from the gasket to the float body. Float level is easily adjustable by bending the float tabs. | 
For high-horsepower applications, Edelbrock machines fuel inlets on both sides of the carburetor. Transforming one of these units into a dual-feel configuration is as easy as removing a plug and screwing in a second fuel inlet. | 
The rate at which the secondary air valve opens can be adjusted by rotating the spring-loaded tensioner after releasing the set screw (left). Rotating it counterclockwise slows the opening rate, while rotating it clockwise speeds it up. |