Carburetor Classic: The Carter AFB – A Closer Look Into A 64-Year Veteran

If you’ve ever driven a carbureted car from the 1950s through the 1970s, chances are pretty good that it had a Carter AFB under the hood.  Introduced in 1957 and still being built today (sold as the Edelbrock Performer), the AFB has performed yeoman service under the hoods of Chrysler 300s, Pontiac GTOs, Lincoln Continentals, and even my personal ’65 Buick Skylark, among myriad others.  AFB stands for “Aluminum Four Barrel,” which is as apt a descriptor as any, and its simplicity and reliability have made it a popular choice of OEMs and hot rodders alike.

Photo courtesy of Super Chevy magazine

Carter was founded in 1909 by Will Carter, but he was apparently less of a businessman than a builder of carburetors, and the company was sold in the 1920s.  Nevertheless, his company soldiered on without him until 1985.  The AFB was not Carter’s first attempt at a four-barrel – the WCFB (Will Carter Four Barrel – such creative nomenclature!) was introduced in 1952 as a standard feature on the Buick Roadmaster (although Roadmasters could also come with a Stromberg four-barrel).  Eventually, Carter produced hundreds of versions of the AFB, working closely with the manufacturers to build one of the smoothest running, reliable carburetors ever.  It was offered in many different sizes, from 400 CFM to 800+ CFM, both in factory specification and in the aftermarket, and it was even used in pairs on cars like the 1965 Buick Riviera Gran Sport and on engines like the Chrysler Street Hemi.

One of the reasons for the AFB’s continuing popularity is its simplicity of operation and ease of repair.  At this point in my life, I can have an AFB or AFB clone (Edelbrock) totally disassembled on my bench in 20 minutes.  Most of the gaskets can be reused if one is careful with disassembly.  Using the 1965 Buick service manual, let’s examine the operation of the AFB.

The AFB is shaped far differently from a traditional Holley 4150/4160 carburetor, and the mounting pattern can be different, but modern Edelbrock variants are produced with two bolt patterns and will fit on most square-bore manifolds (as compared to the spread-bore variety used with a Rochester Quadrajet).  Base tuning of an AFB is simple, with the large idle mixture screws angled up toward the mechanic.  The screws themselves are cast in a way that a mechanic can turn them by hand if given enough room.  The idle speed screw is right out front, easily accessible.  By opening the throttle all the way (engine off, of course), one can access the fast-idle speed screw.

The Carter AFB is unlike the Holley 4150 in that it has only one fuel inlet (remember that the 4160 has one inlet as well), and like the 4150, it has generous float bowl volume.  The AFB has two floats, one on each side of the main body, and setting float height and float drop is a simple but important procedure when performing a rebuild.  One must also check that the float is parallel to the carburetor lid so it doesn’t stick.

To check the float, one simply inverts the carburetor lid and uses a drill bit of the appropriate specification.

The idle (or low-speed) circuit of a carburetor is probably the most misunderstood; main jetting has little to nothing to do with it.  Any carburetor should be able to idle properly with no main jets installed at all, because the fuel is being metered through an entirely separate circuit.  When vacuum is high, fuel travels from the main well to the idle jet, where it mixes with air from the two air bleeds (one on the side of the primary booster and one on the top) for emulsion.  Think of a straw with a hole in the side and you get the idea – a frothy mixture is required for proper metering, which also passes through an “economizer” jet that further reduces the volume of the mixture in the circuit.

Then, the high vacuum underneath the throttle plate (combined with the atmospheric pressure in the float bowls) causes the emulsified fuel to escape (boil, really) past the idle screw, mixing with the limited amount of air bypassing the throttle blades.  As the throttle opens, fuel begins to flow from the transfer slots (which are located above the idle ports in the throttle bore) as they are introduced to engine vacuum, eventually “shutting down” the idle port.  On the street, your car is actually running on the idle circuit up to roughly 2000-2500 rpm under light throttle operation, which is why a main jet or metering rod change makes little difference at low speeds.

It’s hairsplitting but worth noting that adjusting the idle mixture screws doesn’t affect the air/fuel mixture itself, but simply the volume of air/fuel mixture admitted to the idle ports; only the idle jets and bleeds affect the actual mixture ratio admitted to both the idle port and transfer slots.  To adjust the light cruise air/fuel mixture on modern Edelbrock versions of the AFB, I have altered the size of the idle air bleeds on top of the primary boosters.  Others have their own methods, including using pin drills to alter the economizer jet, but most leave them alone (which is wise in most cases, because you can’t put metal back, as they say).

One feature on some OEM AFBs is the thermostatic valve.  Underhood heat can result in vaporization of fuel in the float bowl, which then enters the carburetor air stream through the bowl vent, causing a rich mixture and idle instability.  At the same time, the hot underhood air becomes less dense, further slowing idle speed.  Therefore, in traffic on a hot day or after a long highway run, a bimetal valve will open when exposed to these rising operating temperatures, leaning out the idle mixture.  Modern Edelbrock Performers have eliminated this feature, and I’ve found that it’s easy to see the result on an air/fuel gauge on a hot day – the mixture can sometimes “go rich” in traffic, with resulting idle instability.

Carter’s claim to fame in the carburetor world, however, has more to do with its ingenious metering rod system than anything else.  Most Carters (all?) incorporated this main metering system.  Carters use engine vacuum to overcome a specially tensioned spring and sealed “step-up” piston.  As vacuum acts on the piston, it is pulled down against spring tension.  Being connected to the piston, the metering rod is pulled downward into the main metering jet, reducing the amount of fuel traveling through it.

When the throttle is depressed, and engine vacuum drops, the piston is pushed up by the spring, pulling the metering rod out of the jet, increasing the amount of fuel available for the engine to consume.  The fuel passes an air bleed that is similar in concept to the air bleeds used in the idle circuit.  As air passes through the main venturis, fuel “boils” out into the inrushing air stream, pre-emulsified by the air bleeds, ready to be ingested and burned in the combustion chamber.  The series of bleeds, jets, and rods ensures that the engine is always operating at a proper air/fuel ratio, and this is why Carter spent so much time working with OEMs, and why there are so many AFB part numbers out there.

Under full-throttle, the benefit of a four-barrel carburetor is that it’s two carburetors in one; the secondary venturis only operate when then the engine needs the extra air-fuel mixture.  A common misconception is that the AFB uses vacuum secondaries, such as the setup on a Holley 4160.  That’s not quite true.  The AFB’s secondaries open via a mechanical connection to the primaries.

Once the secondary circuit is introduced to engine vacuum and airflow, a weighted secondary air valve located between the venturis and the secondary throttle plates begins to open, exposing the secondary venturis and only admitting as much air/fuel mixture as the engine can use.

A pair of attached weights help time the opening of the auxiliary valve and close it once the secondaries are no longer in use.  The size and shape of the weights, and even the angle of attack on original AFBs could be different from engine to engine, and this is one reason why any new off-the-shelf carburetor usually requires some extensive tuning to run as well as an OEM carburetor did.  Even so, the weighted flap is most assuredly not easily adjustable, so those interested in more serious street/strip performance may be advised to step up to the later AVS-style Edelbrock, or original Carter AVS, with its adjustable secondary air flap.

The rest of the secondary system works in a similar manner to the main metering system, aside from the fact that the secondaries use no metering rods, only jets and bleeds that were appropriately sized by Carter and the OEMs.

There are several other circuits that are just as important to a car’s driveability, such as the accelerator pump circuit.  Upon quick application of throttle, an engine experiences a momentary lean lag as fuel struggles to keep up with the fast intake of air.  The accelerator pump moves with the throttle.  On an AFB, the pump cup is usually made of leather and seals against a pump bore.  Using a series of check valves to prevent air from being sucked in when it shouldn’t be, or fuel from being pushed out into the float bowl, the pump meters fuel through a carefully sized orifice into the oncoming primary air stream, keeping the mixture somewhat consistent and avoiding a lean lag.

Photo courtesy of Edelbrock

A common misconception is that a larger fuel “squirter” injects extra fuel into the carburetor throats.  On the contrary, a larger squirter simply injects the fuel more quickly; the volume of fuel is solely controlled by adjusting the accelerator pump arm and/or three adjustments in the linkage, both of which adjust the accelerator pump cup height in its bore.

Modern Edelbrock carburetors use a more modern Viton cup (among other materials), but I’ve found that Edelbrock pumps won’t usually work in an original AFB.  Rebuilders differ on which cup material is best, since some original AFB rebuild kits also offer newer materials, but the original leather cup can be reoiled and used for several rebuilds.  Even the spring tension on the accelerator pump itself is different among carburetor models, and it controls the authority with which the pump forces fuel into the engine.  I’ve personally solved lean lags in modern Edelbrock Performers by using a pump from an 800 cfm Edelbrock in a 500 cfm Edelbrock – the 800 uses a stronger spring.

One of the major benefits of the AFB design is the ease with which a mechanic can “rejet the carb.”  More accurately, a change in mixture strength can be as easy as changing out the metering rods, which can be done in a couple of minutes without spilling any fuel.  Most Carter metering rods for the AFB are “two-step” rods – one diameter for cruise, one for power – although some Carters came with three-step rods for even more precise metering.

Changing the “step-up” spring can tailor the timing of the power circuit and can sometimes solve lean or rich bogging; in fact, the springs are similar to the power valve in a Holley carburetor.  Edelbrock currently sells kits to tailor the carburetor to the owner’s car, and their carburetor manual offers very good tuning recommendations.  Carter once sold “strip kits” to accomplish the same thing, but those are obviously much harder to find these days.

Another benefit of the AFB is its effective choke design.  A modern Edelbrock uses an electric choke (as an option), which can be easily adapted to a classic AFB’s original hot air choke.  Either way, the AFB will usually start a cold engine with a pump or two of the gas, with a reliable fast idle.

In this video, Chrysler introduces its technicians to the then-new AFB carburetor, and it explains many of the details I’ve mentioned, and you get to listen to Tech, one of the greatest technicians of all time.

The Carter AFB is a marvel of engineering.  Although it was eventually superseded by the AVS with an adjustable secondary air valve, and today by the Edelbrock AVS2, which uses annular primary booster venturis, the old AFB just keeps soldiering on in original and aftermarket forms.  It’s rarely the most efficient carb (looking at you, Rochester Quadrajet) or the most powerful (there are Holleys on most race cars for a reason), but it might be the best all-around carburetor ever built – simple, durable, easily tuned, and easily understood, the AFB could last another 64 years.

Author’s note: I want to mention that I referred to Dave Emanuel’s Super Tuning and Modifying Carter Carburetors for a few historical notes and to double-check my terminology.