Yes I think you are doing something wrong. You should have 2 lines. One going from the tank to the carb. The other going from the muffler to the tank. The easiest thing to do is unhook both lines. Add fuel to the carb line. When your tank gets full a bit of fuel will come out of the pressure line to the muffler. The reason you unhook both lines is because as fuel goes in one line air from teh tank can escape from the other.