Holley Accelerator Pump Adjustment - Pumping Plastic The Right Way

On Holley carburetors, there are several systems that meter fuel to the engine. Seemingly simple, the high-speed jets are probably the most adjusted and perhaps misadjusted parts of the carb. A few months ago we examined the Holley's power valve and its part in fuel management. This month we examine the accelerator pump circuit and its adjustments.

The accelerator pump housing is located on the bottom of the float bowl. Four-barrel carbs may have a pump under each bowl. This circuit consists of the diaphragm and the arm that actuates it. This arm lies on part of the throttle linkage, which is activated by a plastic cam on the throttle shaft. Once the arms have moved to depress the diaphragm, fuel is pressure-fed through passages in the metering block. These passages terminate under the squirter. The squirter is held in place by a bolt slightly smaller in diameter than its hole. Fuel then travels through the squirter and is released into the venturi air stream under pressure.

This is the way fuel is fed to the engine when the throttle is abruptly opened and there is not enough airflow to pull fuel through the jets. Without the accelerator pump's shot of fuel, the engine will likely backfire through the carb. Backfiring can damage the power valves in older carbs. Holley has a kit to remedy this situation.

How It WorksThe accelerator pump is a simple diaphragm pump. The diaphragm is made of neoprene rubber. There is a spring inside the pump housing. When the arm depressing the diaphragm that pushes out fuel is returned to the closed throttle position, an internal spring returns the diaphragm, sucking in another charge of fuel from the float bowl through a one-way valve. Then it is ready to go again.

The pressurized fuel from the pump soon arrives at the squirter. A close look at the squirter will reveal a two-digit number. The larger the number, the larger the holes in the squirter. Squirters come in several types. Some just have two holes drilled in them. Others have two small tubes affixed to direct the squirt more accurately.

Then there is the anti-pullover type. Its purpose is to prevent manifold vacuum or intake impulses from sucking fuel out through the squirter at the wrong time. This can occur in some types of manifolds.

The squirter size regulates the length of time the accelerator pump shot lasts. The total fuel delivered by one pump shot will be the same with a large or small squirter. The difference will be in the length of time it takes for the pump shot to exit the squirter. In effect, the squirter is a timing device for the accelerator pump.

Let's go back to the beginning of the accelerator pump actuation. Attached to the linkage plate on the throttle shaft is a small plastic cam. The other end of the pivot arm leading to the pump rides on this cam. The shape and position of this horseshoe-shaped cam controls the accelerator pump in relation to the throttle opening. There are six or more cams available for the Holley. 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.

Bobby Writesman of Holley Performance added this to the mix. When racing, a driver enters the corner by shutting the throttle from wide-open to the idle position. The velocity of flow in the venturi will often continue to pull fuel into the air stream. With this fuel present, less will be needed upon re-acceleration. Tuning in this area can be accomplished by changing the plastic accelerator cams, which will in turn change the profile of the pump's output.

At the pump end of this arm is a bolt adjustment with a spring. First, this spring is a safety measure designed to keep a heavy-handed adjustment from breaking or bending the pump arm. The adjustment bolt is another feature allowing adjustments to the pump operation. Adjusting the bolt head away from the pump arm allows the pump arm to relax the diaphragm more. This allows more fuel to be pulled into the pump chamber. Conversely, lowering the bolt head toward the pump arm will reduce the pump arm movement to the point where the diaphragm will hardly move. This reduces pump volume drastically.

What To Adjust And WhyWhy would we want to adjust pump volume? A larger or smaller engine will want more or less fuel added when the throttle is opened. The larger engine will often have a larger-cfm carburetor, and there will be a need for more pump volume. I might mention here that there are two sizes of accelerator pumps for Holley carbs. They are 30cc and 50cc. Adjusting the bolt that rides on the pump arm will increase or decrease the pump arm stroke. Simply put, this adjustment actually has the effect of determining the capacity of the pump. Use it to adjust the total pump capacity to engine size. Too much or too long of a pump squirt will make the engine lazy for a few moments.

The cam setting adjusts the beginning of the pump actuation. Similar in action to a camshaft in an engine, the ramp shape of the cam determines how rapidly the pump is activated. The cam's lift is also a factor. With the many cam shapes available, the cam's effects can be micro-tuned.

Sometimes the cam is adjusted to the driver's style. If a driver pats the throttle several times through a corner, the engine may be very rich coming off the corner. Here a cam might be used that has a slow opening ramp. Also, adjusting the bolt to reduce pump capacity might help. Too much adjusting in this direction may also result in a lag or dead spot in initial acceleration.

Once the pump is activated by opening the throttle, fuel begins to make its way through a series of passages until it reaches the squirter. Under the screw that holds the squirter is a pin with a pointed end. It is easy to lose this pin when working on your carb on the bench. If you feel the need to change or modify this pin, then you already have too many problems. Believe it.

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. When changing the squirter, notice there is a fiber gasket washer under it. When changing the squirter on a carb that is still on the engine, it wouldn't be a bad thing to have a small magnet on hand. But, of course, I'm sure you won't ever drop the screw inside the venturi.

As noted, there are a number of squirter sizes available. Selection starts with your engine's requirements. A small engine might need only a small squirter, maybe a No. 14. The number will be stamped on the squirter body. The larger engine might need a larger squirter, such as a No. 35. Yet there is no hard and fast rule. Remember-the squirter is a timing device. The driver's style and the selection of engine components will determine what is right.

When To AdjustThere are several basic ways an engine reacts when the throttle is suddenly cracked open. Each of the aforementioned adjustments will have effects on this. The thing to remember is to keep a balance.

Your race car is in the shop with the engine idling. You reach over and crack the throttle open. You are surprised. You didn't know there were several Roman candles stored in the intake. When the engine backfires like this (unless the ignition timing is way off), it is lean. Lean means there is not enough fuel to mix with the air for proper combustion. This lean condition should be momentary. Opening the throttle a bit more slowly should cure the problem.

Since you will want to open the throttle quickly, you need to fix the problem. Let's start at the squirter. Check the number. A larger number means the squirter has larger holes. Go up a few sizes. If the engine initially responds well but then has a slight lag, there are several things to look at. This lag could be a result of too much pump squirt for too long, causing a momentary rich condition. Here you might adjust the bolt that pushes on the pump arm. Tightening it down allows more pump stroke; loosening it shortens the stroke. Shortening the stroke reduces pump output.

If this hasn't cured the problem, look at the pump cam. Select one that will open the pump quickly. This, in combination with the bolt adjusted to reduce pump output, will give you a quick, short squirt. If this doesn't work, then go the other way. Open up the pump volume. Then select a cam that will open more slowly, thereby increasing the duration of the pump output.

ConclusionsThere aren't many "chiseled in stone" setups here. Engine component selection, driver, and even car weight all affect the engine's immediate needs. Jets and power valves have great control over running characteristics, but little over immediate response. Learning to tune throttle response with the accelerator pump circuit will make the car more drivable and therefore faster. Quoting Bobby Writesman, Holley Performance Products' manager of oval track motorsports: "A dead or flat spot when opening the throttle is usually a lack of fuel injected by the pump. A flutter along with sluggish acceleration most likely signals too much pump shot."