<h2>How and Why to Adjust a Holley Carburetor Accelerator Pump</h2>
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How many times have you seen or heard a car try to take off when the light goes green, and for some unexplainable reason, it hesitates, or worse, stalls? The reason might not be unexplainable, and nine times out of ten it is caused by a simple adjustment being out of adjustment.
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In order to get an engine to accelerate smoothly, when you push down on the throttle, you need both a>n advance in ignition timing and an increase in fuel delivery. The vacuum advance on your distributor senses when you open the throttle and then provides the extra timing advance that is required. But it is the duty of the accelerator pump on your carburetor to provide the momentary increase in fuel flow.
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When you open the throttle the airflow through the carburetor venturi immediately increases, but since fuel is denser than air, it takes a brief moment for the fuel to be drawn into the venturi. This results in an overly lean air/fuel mixture causing the car to hesitate (stumble) when launching. To compensate for this situation, your carburetor is equipped with an accelerator pump that feeds fuel to the venturi when this condition is encountered. The accelerator pump is connected by a linkage to the throttle butterfly shaft. When you accelerate, the accelerator pump squirts a spray of fuel directly into the throat of the carburetor. This overcomes the momentarily lean condition by increasing the fuel ratio for just a moment.
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Differences in vehicle weight, transmission, and rear axle ratios will affect how much fuel (and the delivery rate of the fuel)  should be provided by the accelerator pump. This may necessitate the customizing of your accelerator pump actuation to meet your vehicle’s needs. Typically, a simple adjustment can cure an improperly functioning accelerator pump.
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On a Holley carburetor, the accelerator pump is located on the bottom of the float bowl. The accelerator-pump system consists of three main components: the pump diaphragm, the pump cam, and the pump nozzle (squirter). If you feel that your accelerator pump needs adjustment, begin by first checking the clearance between the pump operating lever and the pump diaphragm cover’s arm. Do this at at wide-open throttle, the clearance should be around .015 inch. This clearance is to make sure that the pump diaphragm is never stretched to its maximum limit at wide-open throttle. Next, you will want to make sure that the accelerator-pump arm starts to move the very moment that the throttle does.
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The amount of fuel actually being delivered by one accelerator-pump stroke is determined by the pump’s capacity, and the profile of the pump cam. While some might think that a larger squirter will deliver more fuel, this is not correct. An accelerator-pump system is either 30 or 50cc, period. The squirter size affects the time that it takes for that fuel to be delivered by controlling the actual flow into the venturi. A larger squirter allows delivery of the fuel much quicker than a smaller squirter nozzle. If you notice that your car first hesitates and then accelerates, it’s a sure bet that the pump-nozzle size should be increased. If a backfire (lean condition) is experienced when accelerating, this also calls for a larger pump-nozzle size. Conversely, if the car does not feel crisp or clean during off-idle acceleration, the pump-nozzle size may need to be decreased.
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Once the proper squirter size has been selected, the accelerator-pump system can be further tuned by the pump cam. Holley offers an assortment of different pump cams, each with different lift and duration profiles. The cam profile affects the actual movement of the accelerator-pump lever. Changing a pump cam is done by loosening one screw, removing the old cam, and then placing the new pump cam next to the throttle lever, and tightening the screw. Sometimes, there are two or three numbered holes in each pump cam. When inserting the screw in number one, this activates the accelerator pump early, allowing full use of the pump’s capacity. In general, most vehicles that typically run at lower idle speeds (600 or 700 rpm) find this position more useful, because they can have a good pump shot available right off this relatively low idle. Screw positions number two or three will relatively speaking, delay the pump action. These two cam positions are usually better when used on engines that idle at 1,000 rpm or higher.
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We also need to discuss the actual accelerator pump. Like we said, there are two sized 30 and 50cc. The larger accelerator pump is sometimes needed on the secondary side of the carburetor when the carburetor, intake manifold and/or cylinder head flow is too large for the engine. These conditions can cause a weaker signal at low engine rpm, which requires more time for the booster venturi to pull fuel through the main jet. Generally, this is noticeable when a good initial throttle response is delivered, but is immediately followed by the engine bogging down, or flat lines, and as rpm increase, it recovers accelerating normally. That bog or flat is where the pump shot duration ran out, and the booster venturi fuel flow was not enough to keep the engine running efficiently. In this case, the larger pump capacity will help during the transition. While this is a more popular change among foot brake cars, the 50cc is also sometimes a necessity with large plenum 2 X 4 tunnel ram applications especially when racing on a .400 Pro Tree.
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By the way for you curious types, the 30 and 50cc capacity is the volume of fuel that is displaced from the accelerator pump in 10 complete strokes of the pump arm. So the actual capacity is 1/10 that number or roughly 3 and 5cc respectively when opening the throttle from idle to wide open. The 50cc pump is easily recognizable by the large pump arm with the encapsulated spring. The 50cc pump also uses a larger pump cam; either brown (conventional carburetors), or a yellow color (model 4500).
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A 50cc pump will not deliver the 50cc rated volume if any other cam is installed.
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The accelerator pump function is based on hydraulic principles. Therefore, it is essential that the accelerator pump actuation arm/spring always has tension while at idle position. Any air gap here will delay the actuation of the accelerator pump system, allowing air to rush into the engine without fuel, causing a lean stumble/hiccup.
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To change squirters, simply remove the pump nozzle screw, lift off the old nozzle and install the new one. Be sure that both gaskets above and below the nozzle are replaced. If the gaskets are a different thickness, the thicker gasket goes under the nozzle and thinner one on top. The Phillips head screw that holds the squirter in place is special. Under the tapered head, it has a large body section. There is a somewhat fragile tapered washer under the head. Some squirter screws are hollow, and some are not.
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Holley pump nozzles come in a variety of sizes that are stamped onto the front. Typically, heavier vehicles with low numbered axle ratios (like 3.23) will use a smaller nozzle. Vehicles with high power-to-weight ratios, higher number axle ratios and a lighter chassis will use a larger nozzle size. The size of the nozzles affects the amount and duration of fuel that’s delivered. Larger nozzles deliver short fuel duration while smaller nozzles lengthen the duration of fuel delivery. The overall objective is to deliver the right amount of fuel. You don’t want to deliver too much fuel too soon, or to little fuel too late. That’s what causes the bogging and poor throttle response.
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Pump cams from Holley that are available under Holley pn 20-12. There are two screw holes in each one. Each is a different color to aid identification. There is one hole in the throttle plate. The cam can be attached through either one. However, each attachment position will have an effect on the action of the pump. There are 10 cam positions that can be used. These will affect the discharge curve of the pump operation as well as time it to the throttle opening position.
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Sometimes, there are two or three numbered holes in each pump cam. When inserting the screw in number one, this activates the accelerator pump early, allowing full use of the pump’s capacity. In general, most vehicles that typically run at lower idle speeds (600 or 700 rpm) find this position more useful, because they can have a good pump shot available right off this relatively low idle. Screw positions number two or three will relatively speaking, delay the pump action. These two cam positions are usually better when used on engines that idle at 1,000 rpm or higher.
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The amount of fuel actually being delivered by one accelerator-pump stroke is determined by the pump’s capacity, and the profile of the pump cam. While some might think that a larger squirter will deliver more fuel, this is not correct. An accelerator-pump system on a standard Holley Carburetor is either 30 or 50cc.
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In regards to the accelerator pump adjustment, adjust the arm so that you have zero lash at an idle and make sure the pump arm spring is not compressed at WOT. Generally speaking, you want at least .010-.015 inch of remaining lever travel to make sure you are not bottoming out the diaphragm.
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