Virtually every type of carburetor imaginable has an accelerator pump that goes to work as we open the throttle. As we step on the gas, the accelerator pump sprays raw, liquid fuel into the throttle bore to temporarily enrich the fuel mixture for acceleration. If we opened the throttle without using an accelerator pump, the mixture would become too lean and the engine would stall, or at the least, fall flat on its face. This short-term, fat mixture keeps combustion going as we transition from the idle circuit to the power circuit (each to be explained shortly).
These are accelerator pump...
These are accelerator pump nozzles in an Edelbrock carburetor. Note that each directs liquid fuel into the venturi (throttle bore) as the throttle plates are opened. Nozzles vary in size and length depending on the need.
With Holleys, Autolite 2100/4100s,...
With Holleys, Autolite 2100/4100s, and Barry Grants, accelerator pump tuning is also controlled by pump-cam selection, positioning, and pump-cavity size.
We tune accelerator pump performance by controlling how much fuel is sprayed as the throttle opens. Too much fuel can sometimes be as bad as not enough. With too much fuel, we drown out combustion and choke the guy behind us with a thick, fat-mixture fog. What's more, performance falls flat because we nearly put the fire out.
Accelerator-pump performance has to work hand-in-hand with the opening throttle and engine speed to be effective. That's why carburetor manufacturers give us the means to tune accelerator pump performance, and Holley and Barry Grant offer different accelerator pump cams (each adjustable) for tuning purposes. We can not only control how quickly the pump shot happens, we can also control how much. Besides how quickly and how much, we can also control the spray pattern in the bores with different nozzles.
Carburetors are designed with two separate circuits for idle and off-idle performance venues. At idle, we need to deliver fuel to the throttle bore a different way than when we're on the gas with the throttle open. At idle, the throttle is open just a pinch in order to keep intake-manifold vacuum high and allow air flow. We use this vacuum to draw fuel into the intake manifold via the idle circuit in the carburetor. Think of this approach to fuel delivery as sucking a soft drink through a straw. Liquid fuel doesn't burn very well, if at all. This is why we need air bleeds to help vaporize the fuel at idle. Fuel is delivered to the engine just underneath the throttle plates at idle.
Another type of accelerator...
Another type of accelerator pump found in Edelbrock, Carter, and Rochester carburetors is this spring-loaded design that allows the flow of fuel through the nozzles at a predetermined rate. No matter how much you mash the gas, this design allows only a predetermined amount to flow.
This is the idle circuit in...
This is the idle circuit in a typical carburetor design. In its most basic form, the idle circuit needs help to allow an engine to idle. At idle, fuel is drawn from the fuel bowl up to the top of the carburetor as shown. The mixture screw at the throttle plate controls how much fuel flows through the idle circuit.
As we open the throttle, the...
As we open the throttle, the air flow past the throttle plates changes. There are two idle ports, one below the throttle plate and one above. At idle, we are on the idle port below the throttle plate. As we come off idle, we transition from the idle port below the throttle plate and draw fuel from the idle port above. This keeps the fire lit until we move into the power circuit.
When we begin to open the throttle, two circuits come into play on the way to the power circuit. The accelerator pump provides a rich fuel spray as the throttle plates open. However, we also need a smooth transition from idle to power circuit, which comes via the off-idle ports located just above the throttle plates in the throttle bore.