The accelerator pump in a carburetor is responsible for providing additional fuel to the engine when the accelerator pedal is depressed. This is necessary because when the accelerator pedal is pressed, the engine needs a sudden and large increase in fuel to meet the increased demand for power.
When the accelerator pedal is depressed, it pushes a lever in the carburetor that activates the accelerator pump. The pump then sprays a shot of fuel into the carburetor's venturi, which is the constriction in the carburetor's throat that causes the air to speed up and create a low-pressure area. This low-pressure area pulls fuel from the carburetor's float bowl and into the engine. The amount of fuel that is delivered by the accelerator pump is determined by the size of the pump's nozzle and the length of time that the pump is activated.
There may be times when you need to provide measurements of your accelerator pump. Measuring the pump bore (pump well) will be of no help as most accelerator pump wells are tapered at the bottom and the pump will be loose until it reaches the tapered part.
Warm your engine up.
Remove the air cleaner.
Turn the engine off.
Make sure the choke valve is open.
Look down the carburetor and pump the throttle. You should see 1 strong squirt from a one barrel carburetor, 2 squirts from a 2 barrel & 4 barrel carburetor.