Adjusting this bolt changes...
Adjusting this bolt changes the volume of the pump shot. It can be adjusted downward to the point that the pump arm will barely move. Adjusting the bolt upward will allow the arm a longer stroke. The pump will then reach maximum volume.
Why 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.
The accelerator pump is disassembled....
The accelerator pump is disassembled. The diaphragm is activated by the pivoted arm. When the diaphragm is depressed, fuel is pressure-fed to the squirter. When the arm returns to the idle position, the spring forces the diaphragm back, thus pulling in another charge of fuel.
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 down inside the venturi.
This squirter is shown with...
This squirter is shown with two different bolts. While the bolt in the background is hollow with a cross-drilled hole, it is not necessary for gas circle tracking. Use it only when large squirters are necessary. The Phillips head bolt allows fuel to pass around it. When replacing the squirter, take care with the tapered washer used under the head.
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. Driver's style and the selection of engine components will determine what is right.
There 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.
Holley makes three types of...
Holley makes three types of squirters. Some have drilled holes while others have brass tubes inserted. They all have a small number stamped on them. The larger the number, the larger the squirter. The boxy-looking one is an anti-pullover type. This is used when intake pulses in the manifold are strong enough to suck fuel through the squirter even when the accelerator pump is not activated.
Since on the track 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.
There 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."
Changing the squirter with the carb on the engine requires a bit of care. Keep an eye on the gasket position below the squirter during installation. A magnet comes in handy when you drop the bolt in the venturi. Don't even ask how I know that.