The accelerator pump system, a clever mechanical system within the Weber carburetor, delivers a brief squirt of additional fuel during quick acceleration, which can prevent engine hesitation and flat spots.
The system is designed to smooth out transitions when the throttle position changes promptly and is essential for a quality initial startup. Comprising of a flexible diaphragm, piston, spring, fuel chamber, and fuel channels, the system responds to the accelerator pedal, sending extra fuel to the runner via the discharge nozzles above the venturi.
As these diaphragm and spring packages age over decades of use, they can crack, leading to a reduced overall effectiveness. These items should be treated as you would a gasket or a seal - they have a limited lifespan and are in constant contact with petrol.
If you have other seals and gaskets that have aged heavily, consider purchasing a 32/36 Weber service kit.
These Weber accelerator pump diagrams are Sold each.