An accelerator pump is a device found on a carburetor that sprays a controlled amount of gasoline every time the throttle linkage is fully depressed. The amount of fuel introduced into the intake when the accelerator pumps is sufficient to start the engine. The other purpose of the accelerator pump is to dispense additional fuel into the engine when the engine is revved up in certain situations, such as when passing another vehicle. This extra fuel prevents the engine from missing or stumbling until the fuel system can catch up with the needed fuel flow requirement.
In a fuel injection system, there is no accelerator pump. In this type of system, the vehicle's on-board computer senses the need for additional fuel and sends a signal to the fuel injectors to spray more fuel. Typically, carburetor-equipped vehicles do not incorporate a computer or fuel management system in the fuel system. The amount of fuel dispensed when the pump is activated is controlled by a plastic cam situated in the carburetor's linkage system. By manipulating the cam, the quantity of fuel dispensed by the pump can be adjusted from a small spritz to a large squirt.
The accelerator pump is located on the bottom of the fuel bowl of the carburetor. On a typical barrel system, there will be a single pump. On four-barrel systems, there can either be one or two pumps depending on the size and style of the carburetor. On a single-feed or single fuel line-equipped carburetor, there will typically be a single accelerator pump. On a duel-feed or a two-fuel supply hose-equipped double pumper carburetor, there will be a double pump consisting of a pump on both the front fuel bowl as well as the rear fuel bowl.
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On a double pumper-equipped carburetor, the first or front pump is actuated when starting the engine as well as when revving the engine to half throttle. The second pump is used when the rear butterflies or throttle plates of the carburetor are opened rapidly. This is done to prevent a lean condition from occurring, as the flood of fresh air enters the carburetor from rapidly opening the rear throttle plates. On a single pump-equipped carburetor, the added fuel is dispensed through the front pump.
The leading cause of failure for an accelerator pump is a torn rubber diaphragm within the pump. Age, frequency of use and linkage misalignment can all lead to a torn pump diaphragm. Replacement of the pump is a minor repair and typically requires the removal of only four screws.