Subaru Problems? Ask a Mechanic for Answers ASAP
The ball would be a check valve for the accelerator pump.
It would go down the accelerator pump well followed by the spring and pump plunger.
Does your carb look like this?:
Does your instruction sheet for your gasket kit show where the balls go?
Can't find much, you might need a manual to get a better picture.
Or maybe this picture:
Sorry can't be any more help.