Carter AFB Metering Rods & Jets

I get a lot of questions about the AFB and how the jets and metering rods are related, so here we go.
Carter AFB

The AFB, 4 barrel carburetor has 4 jets. Two matching primary and two matching secondaries. The Weber marine carburetor is an exception to this. On that carburetor the primary jets can be of different size and the two secondary jets will be a different size. The primary jets are usually a bigger size than the secondary jets. This is because the metering rods are used on the primary side of the jets, so the jets need to be bigger. This is the opposite from most 4 barrel carburetors, where the secondary jets are bigger than the primary.. The primary jets supply fuel for idle and 90% of part throttle. A two step metering rod is used to control the fuel on the primary side. The larger diameter of the rod is held in the jet at idle and low throttle when the vacuum is the strongest. As the throttle is opened and vacuum drops, the metering rod raises up and the smaller diameter is in the jet allowing more fuel to flow. AFB jets are usually stamped with the jet part number where 120- is the part number and the last 3 numbers is the jet size. 3xx is interpreted as .0xx. Replace the 3 with a 0, i.e. .088. When it is 4xx, replace the 4 with a 1, i.e. .102. When you can’t read the jet you can use drill bits to measure the size. Use a go, no go to accurately nail the size down. Since the metering rods can be changed without taking the carburetor apart, they are usually the better choice when trying to adjust the lean, or rich condition. Change the metering rod one size at a time until you reach the desired results. If you don’t get what you need using the last size metering rod, then you will need to consider changing the jet size. To get to an approximate size relationship between the jet and the metering rod subtract the rod diameter from the jet diameter of the last combination, then subtract that total from the new jet diameter, which will then give you a new metering rod diameter. Example: Jet is .098 and the rod is .076. .098-.076 is .022. To go leaner you select a jet size of .096. .096-.022 equals a .074 rod size. The exact area calculation would get your a .068 rod size.During part throttle operation, manifold vacuum overcomes the tension of the step-up piston spring and pulls the step-up piston and assembly down, holding the large diameter of the step-up ord in the main metering jet. Fuel then flows through the jet and around the metering rod at a reduced volume. During acceleration and under load, the tension of the spring overcomes the pull of vacuum under the piston, the step-up rod will move up so its smaller diameter, or power step is in the jet. Fuel then flows through the jet and around the metering rod at a higher volume.

AFB Jets

In the illustration A indicates a bleed which prevents a rich condition and bog when the high speed circuit is reinitiated after deceleration.The wrong step up piston spring or using a spring that has been stretched out of shape can eratic carburetor behavior at low and high speeds. We sell the metering springs in full sets, which allows you to experiement. For the best running conditions at both low and high speeds, use the lightest spring that you can and still get the best performance from your carburetor. A couple of other things to look for when you aren’t getting the performance you expect is a clogged air bleed or main vent tube, which would cause a rich condition. A float with an incorrect setting will also cause poor performance at higher speeds.

Buy your metering rod springs here.
AFB Metering Springs

AFB Jets

Stromberg WW Throttle Body

Stromberg WW carburetors are notorious  for having worn throttle shafts. Unfortunately unlike most carburetors, the WW wears out the shaft instead of the throttle body.  As I mentioned, most carburetors wear the throttle body and while not a simple process, re-bushing the throttle body on these carburetors is very doable as long as you have the required tools, like reamers and good drill bits. We used to be able to get new throttle shafts to replace the worn shafts, but they haven’t been produced for several years now and finding a used donor carburetor is very difficult unless you happen to find one that has been sitting on the shelf for several years.

To our rescue is someone that I have consulted with many times in the past, “The Old Carburetor Doctor”. Jeff, the owner, has a fix for this problem, which he does when rebuilding the Stromberg WW. He tells me that he will repair your throttle body without a complete rebuild if that is what you want. He just asks that the throttle body be stripped down completely.

Here is what he says about this problem.

Stromberg WWs are no problem. Yes, the shafts are always worn, but we have developed a fix:  I plug the (usually) free right-hand end of the shaft bore with a welch plug, and I rebush the left-hand (throttle lever) end with an extended bushing which allows the un-worn part of the shaft outside of the casting to be supported by new bushing material.  Just look at a WW and you will see how it accommodates this fix; the factory should have done it that way in the first place!

To have your WW rebuilt, you can call Jeff at 800-945-2272. Please do
not call Jeff for technical advice. That isn’t what he does. He is a
carburetor restoration expert. Leave your technical questions here on
this site. Jeff does not sell parts of any kind.

Stromberg WW Carburetor Parts

Holley 1931 Carburetor

The Holley 1931 carburetor is a one barrel downflow type of carburetor. The carburetor is a one piece main body and throttle body casting. The 1969 model had limiter caps installed over the idle mixture screws so that they weren’t tampered with. These can be removed, although I’m not sure about California cars. Cars in this state are under some pretty stringent rules. The 1931 was used on American Motors 1964-68, 199″ and 232″, 6 cylinder engines.

The Holley carburetor number and perhaps the American Motors was stamped on the top of the fuel bowl. Holley carburetor numbers are usually something like R-3966-1A, but you will not find this on the carburetor. R simply means carburetor, The A means assembly. The -1 means it was modified from the original. For example they may have added a vent to the float bowl. Anyhow the list number in the above example is 3966 and will usually be enough as far as finding the correct carburetor parts.

The Holley 1931 can have one of two accelerator pump diaphragms. One has a round stem and the other is a flat stem. The flat stem diaphragm has been discontinued (and we bought what was left), so we have a limited number of kits with that particular diaphragm. You can determine which one was flat by simply looking at the stem where it comes out of the carburetor bowl.

Holley 1931 Carburetor Parts

Holley 1931

Holley 1904 Carburetor

The Holley 1904 is a 1 barrel, downdraft carburetor and has three basic parts, the main body, throttle body and the float bowl. The main body includes the metering block, float and the economizer. The float bowl mounts on the front of the carburetor and can be metal, or glass. The glass bowl is becoming a rare piece to find, undoubtedly because they are so easy to break and they are not made any more. Having the float bowl on the front allowed the 1904 carburetor to be designed with a low profile, which accommodated a smaller space between the engine and the hood.While we are fortunate to have carburetor kit coverage for all of the 1904 carburetor numbers, there are few other parts still available. In particular the float and the plastic actuator are very rare these days. We get quite a few requests for these parts, but when we do find these parts, they generally go on other 1904 carburetors. Your best bet to obtain these parts is to find a used carburetor and use it for a parts doner. There are only a few types of carburetors that we will rebuild these days and the Holley 1904 is one of them.

There are some common problems with this carburetor and fortunately we know what they are and how to fix them. Because of over tightening of the float bowl screws, the main body is often warped causing leaks around the float bowl. This can be rectified by heating the body in a vice and straightening the mounts. Another common problem is fuel leaking past the needle & seat, causing a flooding condition and we have a process that will take care of this problem also. Due to years of wear the choke shaft often has excessive end play causing the idle screw to miss the fast idle cam and possibly cause the choke valve to hang. Last but not least is a problem with the metering block being warped and allowing fuel to leak past. This can be rectified by grinding the surface flat, which we do with a disk sander, but a flat file should also work. All of these problems are fixable and we have the cures, which are detailed on our Holley 1904 technical page and on our many Holley 1904 videos. It would be a good idea to review all of this information before jumping in to your rebuild.