How Carburetors Work – Carbin’ It Easy

Don't Know Squat About Carbs?We'll Explain How They Work, How To Make Simple Tweaks, And How To Know The Difference Between Carter, Holley, And Q-Jet Carburetors.

Some stories are so obvious and necessary that it’s difficult to identify the need. Surely everybody knows what a Q-jet looks like and how to tell it apart from an old Carter or a Holley 4150. “Not so fast, Mr. Smarty Pants Editor,” you might say. “Don’t take so much for granted.” Hard to believe, but a carburetor hasn’t been placed atop a new production car or truck since 1988. That’s 20 years ago, folks. To put it into perspective, a kid who graduated high school in 1988, then got a job as a line mechanic at a new car dealer after Vo-Tech training, could conceivably be a 20-year veteran without ever having been paid to turn the idle screws on a Holley. That puts things in a whole new perspective. Then there’s the guy who only knows Holley carb architecture (which includes Barry Grant, QFT, and many other smaller manufacturers), never thinking to try a new Edelbrock or a Sean Murphy Q-jet.

To another point: We’ll be generically referring to carbs here as “Holley,” “Quadrajet,” and “Carter.” There are many manufacturers that use the Holley architecture, so using the term “Holley” is strictly a convenience to keep the reader from getting confused. The term “Carter” is essentially the same case, though current Carter carburetor architecture is strictly all Edelbrock these days. The Rochester Quadrajet is out of production but is currently available from several sources only as a rebuild. Our thanks to Sean Murphy for providing one of his seriously refurbished units.

The idea here isn’t to compare the pros or cons of one carburetor architecture over another, but to do a straightforward walk-around of the three major types and to show you where to “turn the dials,” so to speak. Maybe after reading this, you’ll want to step out of your comfort zone and try something new.-Johnny Hunkins

The last thing the world needs is another carb versus EFI story, so we’ll spare you the rehashed and inconclusive monotony. What the collective hot rodding community can use is back-to-basics carb tech. Despite its perceived simplicity, meeting the changing fueling demands of a motor- from idle to cruising to WOT-strictly through mechanical methods requires a complex device. The maze of fuel and air passages that constitute a carburetor is hardly intuitive in terms of functionality, and basic carb tuning requires understanding how it all comes together. Furthermore, Holley (aka modular), Carter (now Edelbrock), and Quadrajet carbs all take different approaches to performing the same task. To help those less familiar get up to speed, we’ll cover the basics of how carbs work, reveal simple tuning tips, and divulge the differences between major carburetor platforms.

This cutaway of a Barry Grant Demon carb clearly illustrates the atomization process. Fuel from the float bowl passes through the main jet and enters the emulsion tube. The fuel is emulsified by air entering the emulsion tube through the air bleeds.

How Carbs Work
The principle of carburetion is really quite simple. In liquid form, gasoline does not burn and therefore needs to be vaporized. Through an elaborate network of internal air and fuel passages that rely on rudimentary physics, carburetors introduce atomized fuel into the air stream above the intake manifold plenum, which then vaporizes into a gaseous state by the time it reaches the intake valves.

Fuel is pumped into the bowls through the needle-and-seat assembly, and then drawn into the intake through the pressure differential created by the venturi effect. The hourglass shape of a carburetor’s venturi increases air speed in the section where it necks down, creating a low-pressure area. It is this reduction in air pressure that enables fuel to be pushed from the fuel bowls into the intake manifold. “Think of a carburetor as a fuel injection system that operates at 1 psi of pressure,” explains Judson Massingill of the School of Automotive Machinists. “Everyone thinks manifold vacuum pulls fuel out of the carburetor, but since manifold vacuum drops to zero at WOT, it’s the pressure differential that’s doing all the work. Fuel bowls have air vents in them, which means that there’s 14.7 psi (normal atmospheric pressure) of pressure pushing down on the fuel. The venturi effect reduces pressure to about 13.5 psi at the manifold, and that 1 psi of pressure differential is all it takes to push fuel through the carb.” Moreover, boosters in the venturi, which act as a venturi within a venturi, further increases the pressure drop (carb signal) without significantly compromising airflow.

Even so, the fuel droplets would be too large for effective atomization without a means of introducing air into the mix through emulsification, and that’s where the air bleeds come in. Emulsion tubes carry fuel from the fuel bowls to the venturis and preatomize the fuel by mixing it with air channeled in through the air bleeds. The effect is similar to drinking through a straw that has a hole in it. Ensuring thorough atomization, and therefore complete vaporization of fuel, yields more thorough combustion, increased power, and reduced emissions.

Carburetion would not be possible if not for the venturi effect. As air travels through a carburetor, it speeds up in the section where the venturi necks down in diameter. This creates a low-pressure area, which allows fuel to be drawn into the carburetor.

Unlike many industrial motors, the load and rpm a car engine experiences is in a constant state of flux, whether it’s idling, part-throttle cruising, or at WOT at the dragstrip. Meeting these fueling demands through mechanical means require several networks, or circuits, of air and fuel passages. These consist of the idle circuit, primary and secondary circuits, fuel enrichment circuit, and the accelerator pump circuit. As its name suggests, the purpose of the idle circuit is to provide fuel at idle. Likewise, the primary circuit delivers fuel in proportion to the throttle angle of the primaries, while the secondary circuit initiates additional fuel flow once the secondaries kick in. “While it’s true that changing the tune on one circuit can affect another, each circuit can be individually tuned to effectively meet the needs of the engine’s operating range that it affects,” says Victor Moore of Barry Grant. “However, this means that there are often several ways to address a single issue.”

The fuel enrichment circuit, or power valve, adds fuel only at WOT, and is typically found on the primary side of the carb. This allows running smaller jets for crisp cruising and throttle response and adds extra fuel only when needed. At idle and part-throttle, manifold vacuum keeps the valve shut. However, when manifold vacuum drops at WOT, the power valve opens up and adds the equivalent of 7 to 10 jet sizes of fuel. “With a typical Holley, that means you can have 72 jets up front and 80 jets in the rear so it cruises real nice going down the road. But when you go WOT it’s like having 80 jets in the front and back,” Judson explains. “Everyone wants to block the power valve, but if you block it and then go faster, that just means you were 7 to 10 jet sizes too rich in the first place.”

The accelerator pump circuit is akin to a mechanical fuel injection system and is the only circuit on a carb that is not affected by airflow. It is designed to help speed up fuel flow when the intake charge stalls under heavy loads by providing a small squirt of fuel. “People think that when you floor the throttle and the motor bogs, it’s because the carb dumped too much fuel into the motor, but the exact opposite is true,” says Judson. “What’s actually happening is the volume of air entering the carb is so great that the carb signal drops to where the air stalls and no fuel can be delivered. The accelerator pump combats this lean condition by squirting fuel until air speed and carb signal picks up again, easing the transition between light and heavy throttle.”

Carb Sizing
A quick Google search will find several formulas that recommend a specific carb size based on factors such as engine displacement and rpm range. While they’re better than nothing, they offer ballpark estimates at best. “If you have cylinder heads that move a lot of air and a cam that takes advantage of that airflow, you need a much larger carburetor than those charts recommend,”explains Judson. “However, I fyou have factory heads that aren’t very good, you’re better off with a carb that’s smaller than the charts recommend.”

Perhaps the best way to dial in carburetor sizing is through trial and error with a vacuum gauge. While cylinder heads are flowed at 28 inches of water, carburetors are flowed at 1.5 inches of Hg, which equates to roughly 20 inches of water. Consequently, if a motor never pulls 1.5 inches of vacuum, then the carburetor never flows at its peak potential. According to Judson, on a typical street/ strip motor, it’s ideal to shoot for 1.5 inches of vacuum at Wot.

Start with a carb on the small end of the spectrum-such as a 650-cfm unit-hook up a vacuum gauge to the intake manifold, and run the car up to peak rpm. “If it pulls, say, 2.5 inches, you know that the carb is too restrictive,and you need to step up in size,” Judson explains. “Let’s say you ran the same test with an 800-cfm carb and only pulled 1 inch of vacuum at WOT. Then you might have picked up some horsepower up top at the expense of low- and midrange power, in which case you should size back down to somewhere in between.” Furthermore, a carb that is too large won’t show any kind of reading on the gauge, which means it isn’t presenting any restriction in flow whatsoever -most likely resulting in poor low-rpm metering. It’s better to err on the small side than the large side when it comes to carb sizing, but shooting for 1.5 inches of vacuum at WOT will typically yield the best compromise between peak horsepower and driveability.

Jet Sizing
Understanding how jets regulate the air/fuel ratio is rather simple. Larger jets (with bigger numbers) richen the mixture, while smaller jets lean it out. However, jet sizes don’t necessarily correlate to a specific diameter. For instance, a 40 Holley jet has a .040-inch diameter, but a 70 Holley jet has a 0.073-inch diameter. That’s because all jets are numbered based on what they flow, not their drill size diameter., Just remember: The bigger the jet, the greater the flow.

The needle-and-seat assembly and float control the fuel level in the bowl much like a toilet. Fuel enters the bowl through the seat, which gradually raises the pivoted float arm. As fuel level rises, the float arm pushes up on the needle, which slides into the seat and seals off the fuel passage when the bowl reaches maximum capacity.

Judson’s Six Tips For Carb Nirvana
1 “Everyone wants to put bigger jets in a carb because they have a hot rod and think more fuel equates to more power. The exact opposite is true. Lean is mean. The average carb is engineered on the rich side to save you, so fine tuning usually involves removing fuel from a motor, not adding it.”

2 “Bigger isn’t better. Don’t just go out and install a bigger accelerator pump, a bigger shooter, and a bigger needleand- seat assembly. Carb manufacturers already know how much fuel a carb flows, and the factory setup is fine for 99 percent of motors out there.”

3 “People think carb gurus can get more power out of a motor because they make the carb flow more air, but that’s not true. Where they get the power is in flattening the fuel curve from minimum to maximum rpm.”

4 “Never start out by decreasing the jet size, or you’ll risk burning a motor up. First, up-jet and make sure the motor loses power just to be safe, then startreducing jet sizes. As a rule of thumb, go up one jet size, and if you lose power, down-jet by two jet sizes.”

5 “If you’re running at the dragstrip, gauge your losses or gains based on trap speed, not e.t.”

6 “Carbs work great as long as the fuel bowls are full. If your fuel system can’t keep up, nothing you do to the carburetor means a thing, so it’s paramount to have a good fuel system.”

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.

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.

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.


School of Automotive Machinists 1911 Antoine Houston, TX 77055713-683-3817
Sean Murphy Induction 17662 Metzler Ln. #B, Dept. SC NULL Huntington Beach, CA 92647

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