The Carburetor Shop LLC
204 East 15th Street
Eldon, Missouri 65026
Carter Four Barrel Carburetors
Carter produced 4 different styles of four-barrel carburetor. These are, in chronological order of introduction: (A) WCFB, (B) AFB, (C) AVS, and (D) thermoquad. Each will be covered in some detail. Note that in no case does one type replace the previous type. Beginning in 1957, at least two different types were produced in all years through 1984.
This article will ONLY consider carburetors built by Carter up to and including the 1984 model year. I am quite aware that Federal Mogul, after purchasing Carter, produced several models of clones of the AFB, and these also have been produced to be sold by other companies. However, due to MAJOR design changes, these units will not be considered in this article.
Many enthusiasts are still using, or trying to use these Carter carburetors today, often on non-original applications. The two major mistakes made by the average enthusiast are: (A) starting with a carburetor, which is difficult to adapt to the specific application; and (B) believing that most parts within a given style of carburetor are interchangeable.
An example of (A) might be trying to install an AFB carburetor designed for a Chevrolet 8 cylinder onto a Pontiac 8 cylinder (or vice versa). The Chevrolet and the Pontiac engines have two entirely different fuel requirement curves. Often, when I suggest this to a novice, I am told by the novice that he is smart enough to change jets and metering rods, in which case I concede this point, and walk away. BUT, the novice doesn’t consider other items. For example the Pontiac will want a significantly larger idle jet (pressed-in) than the Chevrolet. Yes, if one has the proper set of drills, and knows the correct size, one may drill out the idle jets. But one must also consider air bleeds, idle restrictors, idle bypasses, and especially the secondary airvalve. The angle of attack of the airvalve is much greater for the Chevrolet engine than for the Pontiac engine. Trying to use the Chevrolet airvalve on the Pontiac will result in hesitation or bog when the secondary engages. Trying to use the Pontiac airvalve on the Chevrolet engine will result in a large “soft” spot at the beginning of secondary engagement.
Can this be tuned? Certainly, if one has all of the specialized tools, parts, and the knowledge of which to use.
We have found one has many fewer issues if one simply does a little homework and starts with a carburetor more or less designed for ones application.
Many enthusiasts are disappointed to learn that very few original equipment carburetors have published CFM ratings, when in fact; most of them do not understand the meaning of CFM anyway. This link may help in the understanding of CFM ratings: LINK
The WCFB was Carter’s first attempt at a four-barrel carburetor. It was introduced in 1952. A total of 222 different WCFB models were made in the years 1952 through 1967. The WCFB was discontinued after the 1967 model year. I have seen a number of different meanings for the WCFB acronym. One employee of Carter once told me that it meant “wrought cast four barrel”; and used that terminology for years. However, a close friend who worked most of his life for Carter, chastised me for using this terminology. It seems the official Carter meaning was Will Carter Four Barrel (Will Carter of course being the inventor that started the Carter Carburetor Company). So I now use this designation when asked.
The WCFB is a “square-bore” carburetor; with three major castings: (A) throttle body (early aluminum, later cast iron); (B) bowl (zinc alloy) and (C) airhorn (aluminum).
I have been unable to find ANY Carter-published CFM ratings for any of the WCFB carburetors. However, for the purpose of argument, one may compare throttle bore size and actual venturi size of various WCFB carburetors to other models, which do have published ratings. WCFBs had approximate ratings from about 375 CFM to about 500 CFM.
The Carter WCFB was identified by a tag, which originally was placed under the drivers’ side rear airhorn-to-body screw. Various sources have attempted to positively identify WCFBs by casting numbers. This may be done in a few rare cases; but generally this is unsuccessful. While the various sources MAY be correct with their casting number tables, generally they are unaware that other carburetors may have used the same blank casting, machined in a different manner. Therefore, if one knows the carburetors identity, one would know what castings would be present; but the reverse is certainly not true. The reader should use any such tables with the proverbial “grain of salt”.
While some of the WCFB carburetors had no airvalve, most of the WCFB carburetors have the weighted airvalve to prevent the secondary from flowing until the engine demand was sufficient to require flow from the secondary. There were also a very few WCFB units that had vacuum controlled secondary. These units, like the vacuum controlled AFB carburetors, were (and still are) exceptionally troublesome. Carter discontinued this design after a couple of years.
While there are many categories of parts on the WCFB that look as if they should interchange; two in particular that give many enthusiasts problems are metering rods and floats. The metering rod is an extremely precision device. Most enthusiasts look only at the metering diameters; but do not realize that the LENGTH of the metering step is not the same for all rods. Carter utilized at least a dozen different step length profiles. When interchanging rods, unless one is deliberately changing the step length, one should compare the step length profile of the replacement to the original rod. The floats used in the WCFB are a double pontoon arrangement, connected by an arm. THERE ARE SEVERAL DIFFERENT FLOATS, WHICH APPEAR TO THE NOVICE AS IDENTICAL! Maximum buoyancy of a float occurs when the flat surface of the float arm is perpendicular to the fuel inlet needle. A method used by Carter (also Rochester in their 4G series, and Stromberg in most of their two barrel series) to adjust the fuel level was to move the height of the arm where the arm was soldered to the pontoons. Failure to observe this difference in floats may lead to carburetor flooding issues. Even if the float arm is bent to achieve the specified adjustment, the buoyancy change may cause a flooding issue.
The AFB was introduced in 1957. Unlike the WCFB, there is no controversy in the meaning of the acronym. AFB means aluminum four barrel. Also unlike the WCFB, several of the 505 different AFB models produced from 1957 to 1984 have published CFM ratings. These ratings range from a low of 400 CFM to a high of 950 CFM. The 950 rating was assigned to only one AFB, the so-called “trapdoor” or three-barrel 3636s used on the Pontiac NASCAR 421 super duty engines.
The AFB, like the WCFB, is a square-bore design. However, the AFB has only two major castings, as the throttle body and bowl assembly are incorporated into one casting. The second casting is the bowl cover or airhorn.
The Carter AFB was identified in a variety of methods:
(A) metal tag under the drivers side rear airhorn to body screw
(B) stamped number on the edge of the passenger front throttle body
(C) stamped number in the center of the throttle body on the back side
(D) stamped number on the front of the airhorn
Some have both a stamped number and a tag; others have a tag only.
Like the WCFB, a very few AFB units had a vacuum operated secondary. This proved to be exceptionally troublesome, and Carter discontinued the diaphragm secondary after a couple of years.
Most of the other AFB units have the weighted airvalve; however, this airvalve was omitted on a very few units which are truly mechanical secondary. Most of these were for racing only.
There are three categories of parts, which give enthusiasts issues with interchange on the AFB. These are: (A) floats; (B) step-up rods (often incorrectly called metering rods); and (C) main metering jets. There are at least 5 different floats, which have been used in the AFB carburetors. These differ primarily in buoyancy. There are several different groups of step-up rods. Some have two steps, some have three steps, and a few have four steps. There are at least 2 dozen different step profiles, as well as two different overall rod lengths. In an attempt to reduce erroneous interchanging of rods and jets; Carter utilized what is called the “high-step” main metering jets in the primaries, which have the 3 step rods. Due to the perceived scarcity of the three step rods and the difficulty in tuning three step rods, some enthusiasts have attempted to change out BOTH the three step rods and high step jets for the more common 2 step rods and standard jets. While this may be done for racing (WOT only), the practice is generally less successful on the street, unless one also changes the primary venturi clusters.
Carter introduced the AVS in 1966. Chevrolet used it on the 275 horsepower 327. The acronym AVS meant “air valve secondary”. Basically, the first incarnation of the AVS was an attempt to place the “demand” spring tension secondary on the square-bore AFB style carburetor. From a manufacturing standpoint, much less expensive to produce the spring tension secondary than the weighted airvalve. Chevrolet used it one year, but then dropped the AVS in favor of the Rochester Quadrajet. Carter still thought it could work, and during 1967 did lots of R & D on the primary circuit, utilizing the 3 step step-up rods and some differences in the primary venturi cluster (remember the Clean Air Act of 1966, to take effect with the 1968 model year had just passed). Due to the really precise lower RPM metering characteristics on the revised AVS carburetor (useful for smog emission calibrations), Chrysler purchased them for use beginning with the 1968 model year. These carburetors were quite troublesome, and before the final demise of the AVS after the 1971 model year, the AVS sometimes didn’t last as long on a new muscle car as the first tank of gasoline! Most enthusiasts changed them out, either for pre-smog AFB units or for Holleys. Carter did make one final attempt to market the AVS in 1972, as they offered several aftermarket versions. However, these didn’t sell; if fact, I have never seen one!
A total 55 different models of the AVS were produced, in two different CFM sizes. The ones with both primary and secondary throttle bores the same size are 750 CFM; the others are 630 CFM.
All AVS carburetors that I have seen have been stamped with the identification number on the passenger side front throttle body edge. Some have also had the metal tag.
Since we will not rebuild the 1968 and newer AVS units, I cannot offer any tips on rebuilding.
The thermoquad, or TQ for short, was introduced as a high performance unit in 1969. Carters’ first spread-bore carburetor of their own design (remember, they produced Q-Jets under license from GM), the first production came in two sizes: 850 CFM and 1000 CFM. In 1971, these racing units would be detuned for street use and an 800 CFM version was original equipment on Chrysler 340 engines. The only 1000 CFM was the race carburetor. Chrysler used 800 CFM units on the smaller V-8’s, and 850 CFM units on the 440’s. Later, 520 CFM units were released for use on the 318. A total of 428 different thermoquad models were produced.
The thermoquad, as stated above, was a spread-bore design, consisting of three major castings: an aluminum throttle body; a thermoplastic bowl; and an aluminum airhorn. Obviously, the name came from the thermoplastic bowl. Carter claimed that fuel temperatures in the bowl were reduced from 25 to 28 degrees F. when compared to carburetors of other companies. Reduced fuel temperature translates into higher engine efficiency.
Most of the thermoquads have the identification number stamped on the edge of the mounting flange, in like fashion to the AFB and AVS. A few of the early racing “Competition Series” thermoquads were identified by tag only.
The thermoquad (opinion) is a wonderful carburetor; but one that can give a novice fits for many reasons. First, there are 4 separate and distinct versions of the thermoquad. These are:
(A) the early production Competition Series with press-in jets
(B) the later production Competition Series with screw-in jets
(C) the 1971 Chrysler units
(D) everything else
Do not attempt to interchange metering parts from the 4 groups.
Second, the thermoquad, unfortunately, suffers a poor reputation, which is entirely undeserved. This reputation is because of two issues:
Issue 1 – THERE ARE TWO BODY SCREWS HIDDEN UNDER THE CHOKE PLATE!!!!! The novice removes the 8 visible screws, and shakes the carburetor to get it apart. Obviously, nothing comes apart. He then taps on the throttle body with a plastic hammer, but still, nothing comes apart. Some time about now, thoroughly frustrated, he attempts to pry the body apart using a screwdriver, AND CRACKS THE CASTING! He now takes the carburetor to someone with more experience to acquire a new bowl, and is shown the two hidden screws. Does he admit to his buddies what he has done? Of course not! He tells his buds that the miserable *&^%$# body warped!
Issue 2 – This one gets even professional mechanics, which have not been trained on thermoquads. The very first production of the last group of thermoquads beginning in 1972 used O-rings to seal the wells. This worked well for Carter in testing, but after about 3 months in the field the O-rings began to leak. Carter then replaced the O-rings with what Carter called “Quad X-Rings”. For those that may be unaware, an O-ring is called an O-ring because the item is in the shape of a ring, and if one were to cut the O-ring, one would observe that the cross-section of the material would be in the shape of the letter O. Cutting the X-ring would reveal a cross-section resembling the letter X. I do not know the current ratio, but X-rings used to be approximately 20,000 times the cost of O-rings. Guess what is in that generic rebuilding kit at your local parts store! Rebuild the carburetor with the inexpensive kit, and the carb starts leaking after a couple of months. Use a genuine Carter kit (or one from the car dealer) and no problem.
In addition to the racing carbs (850 CFM 4846 and 1000 CFM 4847), Carter also offered some high performance aftermarket units of 800 CFM. These are: 9800 (GM linkage); 9801 (Chrysler linkage); 9810 (revised GM with an EGR port); and 9811 (revised Chrysler with EGR port). The 9800/9810 are direct Q-Jet replacements. I have one on my own GTO.