For an automobile to get smooth acceleration, advancement in the timing and an increase in fuel flow are required. There is a vacuum distributor in automobile carburettor that senses when you open the throttle and then it provides the extra advance timing. The accelerator pump provides the extra fuel needed for acceleration.
The accelerator pump is usually connected by to the accelerator; when momentary acceleration is required, the pump squirts fuel directly into the carburettor for increasing the amount of fuel-to-air concentration.
Most carburettors contain a small bent brass pipe pointing straight down it-- it is called the delivery tube. When the throttle arm is pulled firmly, a squirt of fuel flows into the carburettor right from the delivery tube.
This increased fuel increases the power of the engine and the automobile gets an instant acceleration. The accelerator pump must be adjusted suitably for optimum performance, and it is wrong to believe that the more fuel the pump injects, the better it is for the vehicle.