Hi!
Sounds as if you have a "Uni-flow" tank set up!
If you have been around long enough you will know that this tank set up is the best there is, next only to a Tettra "Bubbleless" tank!

The Uni-flow" tank uses two clunks! One clunk goes to the carb and the other to the silencer nipple for pressure. The third fuel line goes up and serves as the filler line and overflow line, this line must be shut when you have filled the tank!

The reason for using two clunks in a tank is for getting a more even fuel pressure to the carb! A one clunk tank cannot provide that!