During acceleration, there needs to be additional gas sent to the engine until the main metering system can take over. This is where the accelerator pump comes in.
As the throttle is opened, the pump lever pushes the accelerator pump down. At the same time the inlet check ball is forced down sealing the pump well off from the float bowl. This forces the fuel in the pump well into a 2nd passage up to the pump discharge nozzle. This raises the outlet check ball off of it’s seat and sends a squirt of fuel out of the discharge into the intake. Actually you will get 2 good squirts out of the discharge.
Releasing the throttle causes the pump lever to pull the pump back up. This movement causes some suction in the pump well and with that and the weight of the fuel, the inlet check ball is raised up and fuel fills up the pump well ready for the next acceleration. At the same time, the outlet check seats and closes off the outlet from the pump fuel.
When installing the two checks, tap them lightly to form a good seal around the ball. A leaky inlet check can cause fuel to be forced back to the float bowl instead of out the discharge. A leaky outlet check can cause gas to leak out of the discharge when idling, causing a rich condition.
The accelerator pump has a delayer spring, clip, then a 2nd spring that goes between the delayer spring and the top. There should be a washer between the top spring and the top of the carburetor. The flat stem pump will be a bit different. There is also a WW that uses a straight round stem and may or may not have a retainer groove. The type without the groove simply sits over the pump. Using a washer at the top would also be nice, but not critical.
With the throttle off, the spring(s) are compressed, then when the throttle is pressed the springs push the pump down squirting fuel.
In this particular WW the carburetor kit will have 2 check balls. The larger one will go in the bottom of the accelerator pump well. The smaller ball goes in the bottom of the main discharge. Nothing on top of the check ball.
Fuel enters from the float bowl into the accelerator pump well via the intake checkball discussed above. The weight of the fuel and the suction of the pump moving up will draw gas into the well.
At idle, the pump is positioned at the top of the pump well waiting for acceleration.
Upon acceleration the pump moves down the well. The pressure will force the intake checkball into the hole sealing the inlet and fuel will be forced out the 2nd hole and into the main discharge.
NOTE: polish the pump well with crocus cloth to provide a smoother glide. Today’s fuel has very little lubricant, which diminishes the life of the accelerator pump. Providing a smooth surface will help.
Add 2 drops of oil to the leather pump cup before installing. It may take a while for the pump cup to expand to where it will stay.