Float Adjustment
Before installing a new carburetor or making any changes on one that has been in service, check two things: fuel pressure and the float level. Often overlooked, both have a profound impact on performance. Fuel pressure or float levels that are too low will cause the engine to run lean, too high and the engine will run rich or flood. Check fuel pressure with a gauge, and follow the guidelines for the carburetor and application. Typically there should be a minimum of 4 1/2 to 5 lbs and a maximum of 7 lbs.
When adjusting the floats, there are actually two considerations: float level and float drop. To adjust float level, turn the top upside down and measure the distance from the tip of the float to the air horn gasket; it should be 7/16-inch. If adjustments are necessary, carefully bend the arm on the float with needle nose pliers.
To check float drop, hold the air horn upright and measure from the gasket to the tip of the float; it should be 15/16 to 1-inch. To adjust the drop, bend the tab that rests against the seat assembly.
Idle Speed And Mixture
AFBs and Performers have two idle-mixture screws at the front of the main body, one for each venturi, and an idle-speed screw on the throttle lever side.
To adjust speed and mixture, allow the engine to warm up. Adjust the idle speed to specification with the idle-speed screw, and then adjust one of the mixture screws to get the maximum possible rpm. Turning the screws in (clockwise) leans the mixture, turning them out (counter-clockwise) makes it richer. Readjust the idle speed screw if necessary, and then adjust the other mixture screw. Again readjust the idle-speed screw if necessary. Finally, lean the mixture screws just enough to get a slight (20 rpm or so) reduction in idle speed then readjust the idle-speed screw. This procedure is known as lean-best idle and will keep the plugs clean and provide the best overall idle quality.
One of the first things to verify when diagnosing any carburetor problem, or installing a new one out of the box, is the float level. All the carburetors' other systems require the proper fuel level to operate correctly. | 
Setting the idle speed and mixture is a multi-step operation. First the idle speed is established with the adjustment screw on the linkage. | 
Next the mixture is adjusted with these two screws at the front of the carburetor. There is one mixture screw for each primary venturi. |
Calibrating Parts And Wide-Open-Throttle
One of the unique features of these carburetors is the enrichment system that uses vacuum controlled metering rods. These rods fit into the jets and serve to effectively alter the size of the orifice. When a lean mixture is called for, vacuum acts on the spring-loaded pistons and pulls the large or economy portion of the metering rods into the jets; that effectively makes them smaller and provides a lean mixture. When the throttle valves are opened, manifold vacuum drops and the springs push the vacuum pistons up, which in turn raises the metering rods. Now the smaller or power portions of the metering rods are in the jet, the orifices are effectively larger, and the mixture becomes richer. Metering rods are only used on the primary side of the carburetor, while the secondary side uses jets alone.
To recalibrate these carburetors a variety of individual parts as well as kits are available. The procedure may involve changing the metering rods only (always the first choice), changing jets, or both. In addition, metering rod springs can be changed.