Two check balls come in the kit, but only one is typically used. It is the ball in the power valve which gets replaced, and it is best removed by inserting a punch or other pin through the holes in the valve body while unscrewing the jet from the bottom.
IMPORTANT: a tiny spring goes in between the jet and the ball; don’t lose it upon disassembly! The other two balls in those jets/check valves in the bottom of the bowl are captive and cannot be removed from the assembly. They must, however, be loose or (at the very least) the accel pump won’t work. And it is still a good idea to remove those valve assemblies to clean them and beneath them — if they will come out.
Intake check ball size: 1/8″
Some BB-1 carburetors use a check ball at the bottom of the accelerator pump well. This needs to be loose. If not, clean it until it is. A screen over the check ball to keep the dirt out would be great, but not critical.