AFB Jets & Metering Rods

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
Carter AFB Carburetor Parts

Metering Rod & Jet Sizes

Weber Marine 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.

CARB NUMBER
CFM
PRIMARY JET
SECONDARY JET
METERING ROD 
2441S
.0935
.0635
2448S
.0645 x .058
2479S
.098
.082
2480S
.098
.082
2506S
.098
.073
2507S
.0935
.0748
2650S
.089
.057
.065 x .060
2651S
.089
.061
.065 x .060
2686S
.089
.061
.0645 x .058
2696S
.098
.082
2797S
.089
.635
.0635 x .053
2798S
.089
.067
2799S
.089
.055
2806S
.089
.057
.065 x
2814S
.098
.086
2815S
.098
.086
2862S
.098
.082
2877S
.089
.092
.0635 x .060 x .053
2906S
.089
.089
2925S
.089
.060
2927S
.089
.067
.0635 x .058
2951S
.098
.086
.065 x .055
2969S
.089
.067
2970S
.089
.067
2991S
.089
.067
3103S
.089
.057
3105S
.089
.065
3106S
.089
.064
3108S
.089
.065
3123S
.0935
.0785
3124S
.0935
.076
3125S
.0935
.076
3131S
.089
.057
3133S
.089
.065
3134S
.089
.065
3140S
.089
.065
3152S
.089
.067
3247S
.089
.057
3249S
.089
.057
3251S
.089
.065
3252S
.089
.065
3253S
.089
.064
3256S
.089
.065
3257S
.089
.065
3258S
.089
.067
3259S
.089
.055
3300S
.0935
.0785
3326S
.0935
.0785
3361S
.1015
.0635
.069 x .0635
3362S
.1015
.0635
.069 x .0635
3479S
.0935
.076
3481S
.098
.082
3545S
.091
.081
3574S
.0935
.076
3647S
.086
.081
3648S
.0935
.086
3649S
.0935
.086
3650S
.0935
.081
3651S
.091
.086
3720S
.104
.0635
3804S
.1016
.069
.073 x .063
3855S
.089
3895S
.091
.0785
3896S
.091
.0785
3898S
.091
.086
3899S
.0935
.086
3900S
.0935
.086
3904S
.098
.082
4030S
.0935
.0785
4033S
.094
.079
4034S
.094
.079
4035S
.094
.082
4036S
.094
.085
4037S
.094
.081
4041S
.094
.079
4061S
.063
.075
4119S
.089
.073
4120S
4121S
.089
.073
4122S
.089
.073
4130S
.089
.0689
4131S
.089
.0689
4132S
.089
4133S
.089
.082
4136S
.089
.082
4139S
.089
.1065
4140S
.073
.066X.058
4168S
.096
.086
.072 x .065
4169S
.096
.086
.072 x .065
4170S
.096
.086
.074 x .065
4171S
.096
.086
.074 x .065
4179S
.062
.075
4200S
.089
.068
4204S
.095
.081
4205S
.095
.081
4242S
.092
.080
4243S
.092
.080
4244S
.092
.077
4245S
.092
.080
4248S
.092
.083
4295S
.089
.074
4298S
.089
.068
4299S
.089
.068
4318S
.089
.082
4324S
0.089
.089PS
.071x.061
4325S
0.089
.080PS
.069X.058
4326S
.098
.080
4327S
.098
.080
4331S
.092
.070
4343S
.089
.080
4360S
.095
.081
4361S
.095
.081
4363S
.095
.081
4393S
.089
.089
4401S
0.089
0.089
.066X.064X.055
4402S
0.089
.080P
.069X.058
4413S
.092
.080
4432S
0.089
.092P
.069X.054
4620S
0.089
.092P
.069X.054P
4621S
0.089
.092P
.069X.054P
4635S
0.089
0.089
.068X.065X.055
4732S
.089
.089
4734S
.089
.098
4736S
.089
.098
4737S
.101
.095
4738S
.101
.095
4739S
.101
.095
4740S
.101
.095
4741S
.101
.095
4742S
.089
.089
4745S
0.089
0.092
.069x.054
4746S
0.089
.092P
.069x.054
4933S
.101
.095
4934S
.101
.098
4935S
.101
.098
4936S
.101
.095
4937S
.101
.098
4967S
.101
.089
4968S
.101
.095
4969S
0.089
.092P
.069x.054
4970S
0.089
0.092
.069x.054
4971S
.089
.089
9605S
600
.098
.095
9606S
600
.098.095
9607S
600
.098
.095
9625S
625
.098
.095
9635S
.098
.095
9635S
625
.098
.095
9636S
625
.098
.095
9637S
625
.098
.095
70 x 47
9638S
625
.098
9755S
750
.113
.107
9756S
750
.113
.107
9757S
750
.113
.107
C3VE-A
CJ127
C7VF-B
CJ127
C7VF-D
CJ127

OE Metering Rod Sizes

Carter AFB Metering Rod Sizes
Updated on 04/05/2022

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