[SteveSchu]

That is the fuel tank VENT line. On my 1983 750 it goes from the main fuel tank to the sub-fuel tank.

 

[SteveSchu]

Think of it like the air VENT cap on any can of gasoline. Try pouring the gasoline out with the vent closed, then with the vent opened. Or a bottle pourer that you see stuck in liquor bottles in the bar. The bartender holds his finger over the vent hole to stop the flow of alcohol into your appletini.