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...
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...
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."