i just picked up a 2000 polaris 800 xcr with 1600 miles 100% stock. The thing is mint, no rust, no damage no nothing, except of a few minor things...... mainly the fuel gauge is no working i would like to know if this is a common problem or what not? what usually goes that make the fuel gauge inoperable? where do i start looking?

Mine is not working at the moment either. Not gotten around to look at it yet. First I'll do is check if the fuse is ok , then check/clean all connectors.
Do you have the electrics diagram? If not , you can download it from www.polarisindustries.com

Fuel gauge has a .2 amp fuse in a pink wire near the gauge. This pink wire is attached to the yellow lighting system wire that feeds the dash lights. Make sure this fuse is good.

The gauge is grounded by a brown wire to the brown ground wire that's part of the gauge lighting system.

If your fuel gauge light works, then the problem isn't in that portion of the system. But, still could be in the wires leading from that system to the gauge. You can test gauge with a 12v battery by applying voltage to the pink wire and grounding to the brown wire and see whether gauge now reads. If it does, you aren't getting power/ground thru the wires.

The fuel tank sender has a purple/white power wire attached to the fuel gauge. And a brown ground wire attached to the sender. It too is part of the lighting system ground circuit. Make sure they are attached.
You can probe this purple/white wire with an ohm meter. Attach ohm meter leads to that wire and to the brown sender ground wire. It will read 100 ohms when tank is empty and 0 ohms when full. If there is no continuity, then trace the wires backwards. You'll probably find the problem there. The sender connector may be loose or disconnected or the brown ground wire is broken. The tank sender could also be faulty.

Good hunting.

Phil