The ball is supposed to seat tightly against this hole so all the gasoline in the cylinder goes through the accel pump outlet passages, instead of leaking back through the inlet passages. The technique here is to place a blunt-end punch or piece of rod firmly atop the old check ball and, with the casting firmly supported, give the rod a few sharp raps. You will know you've done it right when the ball "sticks" lightly into place on its seat; don't panic, just use a small screwdriver to pry the ball off the seat, then discard the old ball and install the new one. This re-forms the seat so the new ball can seal well on it.