My Uncle has a 2004 Sportsman 500HO and he is having a problem with it stalling/not starting if he lets it get below roughly 3/4 to a 1/2 tank of gas. Could this be a venting issue perhaps? Any other guess?

 

Check to see if the vent is plugged, I put a small amount of air pressure to the tank and listen to hear air escaping from vent, it is under the plastic on top of the tank and vents at the headlight, if its venting then the fuel pump could be bad, non efi models have a mikuni style pump located above left front wheel.

 

Good suggestions. I will check on the venting first. I suppose I will tell him to let it get to a point where it stalls out and have him take off the gas cap to see if it runs. If that doesn't work I will tell him to try replacing the fuel pump.