The tube connected to the line with the weight inside the tank (called a clunk) is connected to the needle valve or carburetor. The tube connected to the tube in the tank that turns upward toward to the top of the tank (vent) is connected to the muffler. IF you have a third line, it is a separate filling line. Not all tanks use this line. If you have 2 lines you fill through the line that is NORMALLY connected to the needle valve. Overflow is through the VENT line. DO NOT get these crossed as the system will not work.