When the throttle valve is opened quickly, a large volume of air rushes through the air passage of the carburetor; the amount of fuel that is mixed with the air is less than normal due to the slow response rate of the main metering system. As a result, after a quick opening of the throttle, the fuel/air mixture leans out momentarily. This can cause the engine to accelerate slowly or stumble as it tries to accelerate.
To overcome this tendency, the carburetor is equipped with a small fuel pump called an accelerating pump. A common type of accelerating system used in float carburetors is illustrated in Figure 2-18. It consists of a simple piston pump operated through linkage by the throttle control and a passageway opening into the main metering system or the carburetor barrel near the venturi. When the throttle is closed, the piston moves back and fuel fills the cylinder. If the piston is pushed forward slowly, the fuel seeps past it back into the float chamber; if pushed rapidly, it sprays fuel in the venturi and enriches the mixture. An example of a cutaway accelerator pump is shown in Figure 2-19.