Primary Metering
The main metering system on an AFB is controlled by a jet and metering rod on the primary side. The jet is a fixed metering orifice, which regulates the fuel volume by acting as a restriction to fuel flow. To vary the open area of this orifice, and thus the mixture, the AFB carb utilizes a metering rod with two steps. Think of the jet as a hole for the fuel to flow through, and the metering rod as a stick in that hole. If the stick is thick, it will plug quite a bit of the flow area of the jet. The thinner it is, the more fuel can flow through. The AFB's metering rods are profiled with two steps, appropriately called the "lean step" and the "rich step," or sometimes "cruise" and "power."

Under higher vacuum conditions, vacuum acting on the metering rod piston pulls the metering rod down into the jet, aligning the thick "lean step" of the rod with the jet orifice. With the thick part of the rod in the jet, the fuel volume is reduced, corresponding to the reduced area of the jet open to fuel flow. Under higher load conditions, the engine's vacuum drops and reaches a point where it will no longer hold the metering rod vacuum piston against the metering rod spring's load. At this calibrated vacuum level, the spring will raise the metering rod/piston assembly, and the thinner end of the rod will now be aligned with the jet. With the restriction in the jet effectively reduced, more area is open to fuel flow, and the fuel mixture will become richer.

The foregoing description of the primary-fuel metering system shows that the primary-fuel metering is a result of the combined interplay of a few key components. All of these components-the jet, metering rod, and/or spring-can be altered to tune the fuel curve. The jet and the rod must be considered as a combination. First to consider is the WOT (Wide Open Throttle) mixture. At WOT, the metering rod will, of course, be operating on the thinner "power step" of the rod. Richening the mixture can be accomplished with either a larger jet or a rod with a thinner "power step." Rod changes alone can be made without disassembly of the carb, and can alter the mixture within the range of available metering rods. A jet change can be employed to make a larger change in primary mixture, particularly if the tuning process has exhausted the range possible with available metering rod diameters.

There are a tremendous number of combinations of jets and rods possible, and it's easy to get lost in the calibration process when considering the two variables. Quantitatively, the open area of a rod and jet combination can be calculated mathematically from the relative areas of the individual rod and jets being used. Altering the jet will also alter mixture in "cruise mode," while the engine is under higher vacuum conditions and the metering rod is down on the lean step. For instance, if the jet is increased to fatten up the WOT mixture, the cruise mixture will also become richer than the base setting. If the cruise-mixture ratio is to be retained, a metering rod that is thicker at the "lean step" must be substituted. Edelbrock has an excellent handbook, the Performer Series Carburetor Owner's Manual, which includes charts of the relative mixture changes from the base calibration with a wide range of rod and jet changes. For the tuner, this chart is an invaluable reference, graphically illustrating and quantifying the changes to both the "cruise" and "power" mixture for a given combination of rod and jet.