Yep - my light's come on, gone off, come on again and then sometimes it takes movement after bike has been shut off to come on again.
It's a small tank. I assume it's a float sensor or something that's effected somewhat by the sloshing around of gas and whatnot.
There are also MSR bottles you can pack in a back pack and IMS tanks for greater fuel capacity.
Without an actual fuel gauge to watch, you have to do a little math and know what your range is on a tank of gas.
And you have to know what your range is once the reserve light comes on.
Yamaha gave you a handy tool in that once the reserve light comes on a special odometer shows up and alerts you to how far you've gone on reserve.
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2008 WR250X
Gearing: 13t - 48t
Power Commander 5 / PC-V
Airbox Door Removed - Flapper glued - AIS removed
FmF Q4
Bridgestone Battlax BT-003rs