Accelerator PumpVirtually 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).
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.
Idle CircuitCarburetors 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.
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.