Jet Selection
When changing jets, go two sizes; there is approximately a 4 1/2-percent flow difference from one jet size to the next, and one size won't make that much of a difference. If you must go up or down excessively, eight to 10 jet sizes, there is a problem with the fuel delivery system or the carburetor is wrong for the application.
Accelerator Pump
The first adjustment to check is the clearance between the pump operating lever and the pump diaphragm cover's arm; at wide-open throttle, it should be around .015-inch. The purpose for this clearance is to assure that the pump diaphragm is never stretched to its maximum limit at wide-open throttle, which will cause premature pump failure. With the proper clearance established, operate the throttle making sure that the accelerator pump arm is being activated the moment the throttle begins to move. This will assure that pump response will be instantaneous. Adjustments are made with the screw that is located on the accelerator pump arm.
The amount of fuel that can be delivered by one accelerator pump stroke is determined by the pump's capacity and the profile of the pump cam. The period of time that it will take for this predetermined amount of fuel to be delivered is affected by the pump nozzle size. A larger pump nozzle will allow fuel to be delivered much sooner than with a smaller pump nozzle. If more pump shot is needed sooner, a larger pump nozzle size is required.
During acceleration tests, if you notice that the car first hesitates and then picks up, the pump nozzle size should probably be increased. A backfire (lean condition) on acceleration also calls for an increase in pump nozzle size. On the other hand, if off-idle acceleration does not feel crisp or clean, the pump nozzle may be too large.
Holley accelerator pump nozzles are stamped with a number, which indicates the drilled pump hole size. For example, a pump nozzle stamped "35" is drilled .035-inch. Pump nozzle sizes are available from .025- to .052-inch (whenever a .040-inch or larger accelerator pump nozzle is installed, the "hollow" pump nozzle screw should also be used).
As with main jets, when changing the pump nozzle it's best to jump more than one size; in this case go three sizes. For example, if there's an off-line hesitation with a No. 28 (.028-inch) pump nozzle, go to a No. 31 (.031-inch). If a No. 37 (.037-inch) or larger pump nozzle is used, then a 50cc pump should also be installed.
Once a pump nozzle size selection has been made, the accelerator pump system can be further tailored with the pump cam. Holley offers an assortment of different pump cams, each with unique lift and duration profiles. Switching cams will directly affect the movement of the accelerator pump lever, which impacts the amount of fuel available at the nozzle.
Carburetor main jets determine the air/fuel ratio. They look similar to the bleeds that introduce air into the fuel being discharged in the venturis. | 
Included in all the BG kits are high-tech red, non-stick gaskets. They make subsequent disassembly of the carburetor much easier. | 
This is the front of a Demon carburetor body, the variety of passages match those in the metering block. |