1964 & later
How the accelerator pump circuit works:
When you let up on the throttle the check valve opens and fluid fills the pump chamber.
Pressing on the accelerator puts pressure on the check valve to close it and forces fuel through the bottom hole into the main discharge passage and out the main discharge.
With the check valve installed.
The new check valve has a long stem which you insert into the hole, then pull it through until the umbrella is all the way seated. Trim the stem off so that it doesn’t intrude on the float.
4100 Check Weight measurements. Length is important because it needs room to move up when the check ball moves up to let fuel in.
You can buy a new weight here.