Holley accelerator pump
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This page is designed to give enough info for an individual to properly adjust and tune a Holley accelerator pump. Portions have been sourced from various Holley documents.
 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).
 Transfer slot
Also, make sure the carb is not idling on the transfer slot.
The drawing below shows the transition slot as seen with the carb held upside down. The drawing on the left shows an overexposed transition slot. Baseline the throttle blades to give a transfer slot that looks like the image below, right (approximately as long as it is wide, or about 0.020", up to 0.040"):
From this point, if the idle speed is increased to the point where the transfer slot becomes over exposed, a poor idle and off-idle transition can be the result. One sure sign of this is the idle mixture screws do not cause the idle to change when turned in or out. Usually, adding initial timing to increase the idle speed will allow the throttle blades to be closed down some using the curb idle screw. This will return the idle mixture screws to normal and the idle will be smoother and cleaner, as will be the transition from idle to main circuits.
In addition, the secondary throttle blades may need to be opened slightly to allow more idle air (called idle bypass air) to be introduced into the engine.
 Ignition advance
As mentioned above, correct initial advance plays an important part in a clean transition from idle into the main circuits. So the timing has to be sorted out before carb tuning is attempted, else a problem with the timing can seem as though it is a carb-related issue in general, and an accelerator pump issue in particular.
Also, 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.
More on correctly timing the engine can be seen at Ignition advance
 Accelerator pump check valves
Another thing that can delay the accelerator pump action is a leaky check valve.
There are two check valves:
- Under the shooter- usually a pin and weight or sometimes just a pin or just a ball.
- Under the pump diaphragm- this will be a red/orange "umbrella"-type valve (newer carbs) or a "ball and bail" affair used on the older carbs.
 Repairing the check valve seat
Both the check valves have to seal well or there won't be an immediate discharge from the shooter. The check valve seat under the squirter can often be repaired if it has an imperfection that prevents it from sealing properly. If you have a used check ball or 'pin' of the same type your carb uses, you can use it to carefully reform the seat in the carb body casting.
You place the check ball/pin down into the hole, then using a long, thin, flat-ended punch, gently tap the check with a small hammer. This will remove any raised edges or whatnot that may be causing the check to not seat tightly.
Under the pump diaphragm, if you have the rubber umbrella, be sure that it is still very pliable and the sealing surfaces on the carb as well as the umbrella are smooth and defect-free. If there's any doubt at all, simply replace the old check valve with a new one. It's a good idea to do this any time the pump is apart.
If it has the older "ball and bail" style check valve, use the same procedure as for the under-nozzle check valve to re-form the ball seat.
Removing and replacing the bail is no fun, and must be done very carefully so as to maintain the slight bend in the bail. It's best to re-peen the metal to secure it back into place. Only do this repair if it's necessary.
Here is a Holley accelerator pump video
 Shooter assembly
Be sure the two gaskets are in place and not split. Any leakage past the gaskets will result in a less-than-normal discharge at the nozzles.
 Primary or secondary
On a double pumper, the side of the carb (primary or secondary) having the problem can sometimes be found by disconnecting the secondary throttle linkage to allow the carb to run on just the primaries. If the problem associated with the accelerator pump goes away, it can be surmised the problem lies with the secondary side.
This is useful because while a problem with the secondary pump delivery can sometimes be "cured" by going larger on the primary side, going larger than necessary on the primary side hurts the economy and can cause the carb to not be as 'crisp' as it could be, even though the hesitation is gone.
 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.
Note: 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”.
 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 position #1 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 cam charts
- The following chart is from Jason Kolk, as posted at vintage-mustang.com
- This is info supplied by Holley
- Cams from smallest to largest
Holley says: "Accelerator pump shot relative to cam color from lightest to heaviest are as follows":
 Dominator pump cams
There are just two Dominator pump cams that come with the larger throttle shaft profile, as shown below. The other cams can presumably be modified to work on a Dominator.
 50cc pump cams
The pump cams having three screw positions can be used on a 50cc pump, in position #3. Some are only for use with a 50cc pump.
 Pump discharge nozzles, or "shooters"
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.
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 all used accelerator pump nozzles closely to see if they've been drilled oversize. If a shooter has been drilled out, the number on it obviously will no longer apply. You may have a hard time telling, if they did a good job so a numbered drill bit set or a wire spark plug gap feeler gauge will help determine the exact size.
Only drill shooters oversize if it's absolutely necessary. For the best repeatable results use factory shooters. If you find you have to drill a shooter, no big deal, but do yourself a favor and etch the new size onto it so you'll know what it is later on.
- Aftermarket Holley carb numerical listing by carb number
- Holley HP series carb specs by carb number
- Holley's accelerator pump instructions are on pages 6 and 7
- Holley instruction manuals by part number
- Manual, Exploded View and Nomenclaturefor Holley models 2010, 2300, 4010, 4011, 4150, 4160, 4165, 4175, and 4500
- Holley technical library