Generally:
The one on the very rear of the tank is the vent/overflow. If you look under your gas tank theres a tiny hole. If you spill gas there during a re-fill it just goes out that hole at the back.
That vent usually get's "T"-d into the overflow from the carbs.
On the petcock (the fuelshutoff) the big one is where the gas goes to the carbs and the little one is for a vacuum line from the carbs.
When you start the bike the vacuum sucks on the petcock, opening a valve and allowing fuel to start flowing.
The one on the very rear of the tank is the vent/overflow. If you look under your gas tank theres a tiny hole. If you spill gas there during a re-fill it just goes out that hole at the back.
That vent usually get's "T"-d into the overflow from the carbs.
On the petcock (the fuelshutoff) the big one is where the gas goes to the carbs and the little one is for a vacuum line from the carbs.
When you start the bike the vacuum sucks on the petcock, opening a valve and allowing fuel to start flowing.