The carb pressure connections. It does run rough without that balance tube connected. Spent about 2 hrs & 15 minutes (0730 - 0945) getting the idle speed set and balanced. Then balancing again at 3600 RPM. When all balanced at max sensitivity, shut down and reconnected the balance tube. Ran the engine to check it out from idle to 5800 rpm. Smooth as a well oiled sewing machine.