your jets could be gummed up or have dirt particles in them. especially if it sat for awhile. gas these days is junk after a few months of sitting. my suggestion would be to buy a rebuild kit, a jet kit and slap it back together... especially if you have an after market exhaust. DO NOT RIDE IT IF ITS SPUTTERING. that causes the engine to heat up too much and can cause alot of damage. you can actually burn valves super easy. you could even crack the valve seat from too much heat and itll drop the valve which could possibly destroy the piston. if you dont have an after market exhaust, you can pick a stock jet kit.. they are pretty easy to find for all year warriors over 89 on EBAY.
when i first bought my 99, it would sputter through mid range. i never addressed the issue cause i wanted to was ride. which was stupid. i actually had the seat crack on me, which dropped the valve. i got lucky and only bent the valve and jus barely scored the crown of the piston. i bought a carb rebuild kit, jet kit, titanium valves, seat and springs. now she runs like a raped ape