THE CARBURETOR SHOP / Carter AFB tips

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Eldon, Missouri 65026

Carter AFB tuning tips

Since everyone who works on AFB carburetors knows all there is to know concerning tuning the step-up rods, springs, and main metering jets; I will not bore you with explaining these. This article is directed at some of the less-understood tunable facets of the AFB, plus a couple of tips.

This article is concerned only with genuine Carter AFB carburetors. None of the imitations, regardless of what name may be on them, are considered here.

We offer probably the world’s largest selection of parts for genuine Carter AFB carburetors.

The accelerator pump

There are four variables in tuning the AFB accelerator pump circuit. These are:

(1) The diameter of the accelerator pump
(2) The length and articulation of the accelerator pump
(3) The mass of the pump discharge valve
(4) The diameter of the pump discharge nozzles

While other companies found the need for multiple accelerator pumps, Carter simply changed the operating volume of the pump squirt.

Carter used four different diameter accelerator pumps on the various AFB carburetors: 9/16, 5/8, 11/16, and 3/4 inch. Additionally, Carter used a number of different lengths of pumps. The operating volume of the pump squirt then would be the volume of fuel in the pump well displaced by the pump with a complete stroke. Obviously, changing either the diameter of the pump well or the length of the stroke would alter the operating volume.

Back when accelerator pumps could be purchased individually, a few racers would bore the accelerator pump well to accommodate a larger diameter pump. We still occasionally use this trick when preparing an original carburetor for racing. We also inform the owner which kit to purchase in the future that will contain the proper pump. Generally, the pump volume can be controlled by the use of a shorter pump. When installing a shorter pump, it is important to remember that the pump MUST be in lower portion of the pump cylinder that is below the taper.

The duration (time) of the pump squirt is controlled by the diameter of the pump discharge nozzles.

The last variable is virtually always left alone. The mass of the pump discharge valve will effect (in milliseconds) how soon the fuel from the pump actually enters the venturi once the pump is actuated. Carter used an aluminum ball, a steel ball, and several different lengths of square brass needles. Each of these valves would have a different mass. The trick is to make the mass sufficiently large such that the valve is not lifted from its seat by low pressure in the pump discharge nozzle; but small enough that it will move almost immediately with pump actuation.

Pump link installation.

This is the little "S" shaped link that connects the accelerator pump rocker arm to the accelerator pump.

The link is constructed with a bend in the middle.

When properly installed, the link will resemble the letter "S" if viewed from the front, NOT a reverse "S".

If viewed from the throttle side of the carburetor, when the lower portion of the link is vertical, the upper portion will be leaning toward the rear of the carburetor.

I know a picture is better, but my digital camera works best outside, and the temperature is less than optimal.

Installing the link backwards will result in the necessity of much greater pressure to depress the footfeed, and also premature failure of the accelerator pump.

The auxiliary airvalve

One of the least understood issues of tuning the Carter AFB is the importance of the auxiliary airvalve.

Carter supplied many different airvalves in their various AFB carburetors. Since most do not have a ready supply of airvalves, and they are not readily available, the easiest way to tune this variable is START WITH THE PROPER CARBURETOR FOR THE APPLICATION!

The auxiliary airvalve controls the timing of when the air starts to flow in the secondary circuit. The fuel will have already begun to flow through the secondary starter circuit. The airvalve consists of a shaft with two valves, and two offset weights. Since Carter used a variety of weights, most enthusiasts think that the weight is the tunable variable AND NEGLECT TO LOOK AT THE ATTACK ANGLE OF THE VALVES.

Yes, the weights ARE important, but the attack angle is easily as important, if not more so. We have had many carburetors come to us with weights ground almost to nothing, or chunks of lead screwed onto the weights when the angle of the valves was incorrect for the application.
For STREET vehicles (you know, no 9000 RPM start from a stop sign, just spirited driving) GENERALLY, "torquer" engines like a small attack angle; and "screamer" engines like a large attack angle.

Sometime take a look at the angle on an original AFB for a Pontiac, Buick, Chrysler, etc., and then compare it with the angle on the aftermarket AFB's.

Using the large angle as found on the aftermarket (SBC) carburetors on the torquer type engine causes the airvalve to whip open too soon, which may cause a hesitation or bog when the secondary begins to open.

Conversely, trying to use one of the Pontiac valves on a SBC will cause the engine to starve for air, and lose power as the secondary begins to open.

Again, since few enthusiasts have a large selection of the various airvalves, the easiest method of tuning is to start with the correct carburetor. Carter did not build 505 different AFB's just to remove the R & D money from the profits of the shareholders. One size or type does NOT work for all applications.

Step-up rod interchangeability

I am often asked if one can use the newer "imitation" rods in an original Carter AFB.

- The short answer is: "Maybe, maybe not, but I don't recommend it".

- The long answer:
Carter made 505 different AFB carburetors (I don't count the imitations). For these carburetors, Carter produced hundreds of different step-up rods.

Most enthusiasts think that these rods differ in metering diameters only; but this is totally wrong. Carter made 2 step, 3 step, and 4 step rods. They made rods in 2 different overall lengths; but where things really get ticklish is the step length. There are at least two-dozen different physical rod profiles. As an example: given 2 different 2 step rods of the same overall length, one might have a low vacuum step 0.250 in length, a taper of 0.050 length, and a high vacuum step of 0.515 in length. The second rod might have a low vacuum step 0.400 in length, a taper of 0.100 length, and a high vacuum step of 0.500 in length. Some of the carburetors with more than 2 step rods had modifications to the primary venturi cluster to utilize the extra steps.

The step length and taper are crucial to maintaining the "timing" of different metering levels and this is correlated with the tension of the step-up springs.

If one is to use a rod NOT specified by Carter in an original AFB, for best results, one should verify that the physical profile of the rod is the same.

The same philosophy holds when trying to take an aftermarket carburetor (whether genuine Carter or one of the imitations) and recalibrate the aftermarket unit to duplicate an O.E. calibration.

The idle air bleed circuit

Many throw rocks at the AFB's with the idle air circuit (the large screw between the two idle mixture control screws).

I will admit that idle adjustment can be tricky with this system if the user is trying to burn E-10.

The solution to idle quality on "driver" vehicles (not numbers matching) is to install a conventional idle control. There is a "dimple" in the lower casting on the front drivers side (where the normal curb idle screw would be located). Drill through the dimple using a number 21 drill, and tap for a number 10 x 32 machine screw. Obtain a 10 x 32 machine screw and spring at the local hardware or auto parts store. Install the screw and spring. Screw the idle air screw all the way in, and then adjust the idle the conventional way.

If you wish to leave the adjust as the factory suggested with the idle air screw: for MOST applications, the beginning settings would be 1 turn on the idle mixture screws and 1 1/2 turns on the idle air screw.

Where these carbs are really useful is for those using big cams (low vacuum). The idle air screw, used in conjunction with the conventional idle screw is an adjustable idle bypass. Remember the holes drilled by the factory in the throttle plate on some of the performance carbs? No need to drill, as the idle air screw will do exactly the same function, and it is adjustable.

Rebuilding tip

And finally, one rebuilding tip. The step-up pistons should ALWAYS be replaced when rebuilding a Carter AFB. WHY?

Originally, Carter used brass step-up pistons, as they had done successfully for decades in carburetors with zinc alloy (a.k.a. "pot metal"). However, they quickly found that the brass pistons were "harder" than the aluminum body of the AFB, and wore the body in the vacuum cylinders.

So Carter, instead of leaving a situation that would allow them to sell more carburetors, changed the metal used for the step-up pistons to a very soft aluminum. Yes, movement of aluminum against aluminum does create wear; but since the pistons are a much softer material than the castings, the pistons are sacrificial, saving the castings.

Even a few thousandths of wear on the pistons will change the timing of the rod step change. Yes, one can compensate by using weaker step-up piston springs; which would not be necessary if the pistons are changed as Carter intended.