Accelerator Pumps - Pumping Plastic

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 that 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.

The 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 ten 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.