Polish the pump well with crocus cloth so that it is very smooth. Today's fuel has little lubrication which is bad for the pump cup. It needs a smooth surface to glide smoothly.
Most Aisan carburetors have a check ball located beneath the accelerator pump, known as the inlet check ball. When you release the gas pedal, the pump moves upward. This upward movement creates suction, and the weight of the fuel helps push the check ball upward, allowing the pump well to fill with fuel.
When the gas pedal is pressed the check ball is forced back down keeping the fuel in the well. The downward pressure of the pump forces the extra fuel that the engine needs on acceleration to move through a 2nd passage in the pump well and into the main discharge.
