Most carbs I have worked on with the primer bulb and return line,are set up so when you prime the carb, it fills up the fuel cavities in the carb, then the excess is sent back to the tank via the return line. Many fuel caps have a one way rubber valve called a duckbill valve that lets air into the tank, but not back out again, so you can tilt it on it's side and the fuel won't run out of the tank vent.