Editing Holley accelerator pump

==Overview==
Some of the following has been taken wholly or in part from various Holley documents. No claim is made by the author(s)/editor(s) as to this being an original document, although portions ''are'' original.

==Accelerator pump==
The accelerator pump's purpose is to make up for the lag in fuel delivery to enable the engine speed to increase in response to throttle opening. Differences in vehicle weight, transmissions, and rear axle ratios affect the amount of fuel and the delivery rate that should be provided by the accelerator pump. This may necessitate the customizing of your accelerator pump to your vehicle and its use.

==First things first==
Before changing the size of the shooters or the pump cam, be sure the accelerator pump is adjusted correctly- no play and fuel from the shooters the instant the throttle shaft is rotated, and about 0.015" clearance at wide open throttle (WOT).

Also, make sure the carb is not idling on the transfer slot. More on that here.

If there is a vacuum advance being used and it is hooked to manifold vacuum, the engine can have a slight to a major hesitation if the engine is overly dependent on the increased timing from the vacuum advance. In some cases the drop in vacuum and subsequent loss of ignition advance at the hit of the throttle can cause the hesitation. This ''shouldn't'' be the case if the initial timing and the torque converter stall is sufficient, and the vacuum advance amount isn't excessive (limited to about 10 degrees of vacuum advance max), but stranger things have happened.

==Pump cams and shooters==
Two factors that affect the accelerator pump’s delivery is the pump cam and the pump shooter (discharge nozzle). The pump cam determines the total volume of fuel and affects delivery rate; the pump shooter diameter affects delivery rate and has a major effect on the ''duration'' of the shot.

{{Note1}}The old saying “if a little is good, a lot is better” does not apply to the proper tuning of the accelerator pump. Your car’s performance can be just as bad if it receives “too much fuel too soon” as if it receives “too little fuel too late”.

{{Note1}}Pump arm adjustment/clearance should be checked and verified each and every time the pump cam and/or pump cam position is changed.

===Pump cams===
The cams will have 2 (some 3) holes in them. Besides the change to the fuel delivery curve made by changing holes, position #1 is used when the throttle blades at idle are "normal", position #2 is when the tune of the car requires an idle speed of >1000 RPM or so. Position #2 makes up for the additional throttle shaft rotation that's required for the higher idle speed. Cams w/position #3 are usually used on REO 50cc pumps. Generally, vehicles which normally run at lower idle speeds (600-800 RPM) find this position more useful because they can have a good pump shot available coming right off this relatively low idle. Positions #2 and #3 delay the pump action, relatively speaking. These two cam positions are good for engines that idle around 1000 RPM and above. Repositioning
the cam in this way makes allowance for the extra throttle rotation required to maintain the relatively higher idle setting. 

===Pump shooters===
Holley says: "If you need more pump shot '''''sooner''''', then a larger pump nozzle size is required. During acceleration tests, if you notice that the car first hesitates and then picks up, it’s a sure bet that the pump nozzle size should be increased. A backfire (lean condition) on acceleration also calls for a step up in pump nozzle size. Conversely, if off-idle acceleration does not feel crisp or clean, then the pump nozzle size may already be too large. In this case a smaller size is required." If someone can follow the vehicle in question, a too-rich accelerator pump will often cause a puff (or more) of black smoke, indicating a too-rich condition.

The pump shooters have a number stamped on their casting which designates the shooter size in thousandths of an inch, i.e., a #25 shooter has a 0.025” discharge orifice. The smaller diameter nozzles lengthen the pump shot duration and are used with heavier vehicles or with vehicles equipped with lower numerical rear axle ratios.

Larger diameter nozzles (0.035” - 0.037”) shorten the pump shot duration, but deliver a greater initial volume of fuel. These sizes should be used on applications where engine speed will increase rapidly (vehicles with good power-to-weight ratios). Best acceleration is achieved when the accelerator pump delivers the lean best power air/fuel ratio to the engine; not when the maximum volume of fuel is supplied.

If a shooter size greater than 0.038”-0.040” is required, a “hollow” discharge nozzle screw is advised. Above a 0.037” squirter size, a 50cc "REO" pump may also be required, available under Holley P/N 20-11. Make squirter changes in 3-step increments- i.e. if you have a 28, go to a 31, etc.

Be sure the squirter you are using hasn't been drilled oversize! If it has, the number on it obviously will no longer apply.

The smaller sized accelerator pump squirters will cause the duration of the pump shot to be longer. Because the fuel cannot be compressed, the spring in the accelerator pump lever compresses then this spring pressure continues to supply fuel through the squirter until the spring has extended back to its original length.

An important point should be kept in mind when tuning a double pumper: The secondary accelerator pump must supply fuel for a sufficient time so that the secondary main nozzles can begin to flow fuel to the engine after the secondary throttles are opened. If the nozzles do not start by the time the pump shot expires, bogging will result. 

Also check the accelerator pump nozzles closely to see if they've been drilled oversize. You may have a hard time telling if they did a good job so a numbered drill set or a wire spark plug gap measuring feeler gauge would help to determine the size.

Only drill shooters oversize if it's absolutely necessary. The factory finish is better than most home tuners are going to achieve, and this effects the flow to some degree. So for the best results, use factory shooters, and especially jets.

==Resources==
*Holley's accelerator pump instructions are on [http://www.holley.com/data/Products/Technical/199R8108-2.pdf pages 6 and 7].
*[http://www.holley.com/TechService/Instructions.asp Instruction manuals by part number] 
*[http://www.holley.com/data/Products/Technical/199R10014.pdf Manual, Exploded View and Nomenclature]for models 2010, 2300, 4010, 4011, 4150, 4160, 4165, 4175, and 4500
*[http://www.holley.com/TechService/Library.asp Technical library]

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[[Category:Engine]]
[[Category:Carburetors]]