I get a lot of questions about the AFB and how the jets and metering rods are related, so here we go.
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.
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.
Metering Rod & Jet Sizes
This chart includes what we know about the original size and part numbers for the Carter AFB carburetor jets and step up rod (metering rod). Keep in mind that these specifications were written when gasoline was much different than it is today. Use only as a guideline. It’s all about trial & error these days. Since it now takes more gas to go the same mile, fudge to the bigger size jets and smaller size metering rods.