When you have the tank off; give it a good clean out when it is accessible.
The tap handle is best removed from the tap itself ( X head screw down the center and pull off) when removing or refitting the tank.
If not done; sooner or later the brittle plastic tap inner snaps off (you have been warned )
When the tank is off; take note of which tap position is for ; on/off/reserve.
It is in an awkward place and some taps have had the inners swapped in their past life.
Here is an online parts fiche that is nearest to your model;
http://www.cmsnl.com/yamaha-fzr400u-198 ... partslist/
Sent from my keyboard using the English language not some teen text shite from a fooking phone.