I have a 1997 sportsman 500 that i purchased used (very) and cheap. It starts, runs and drives fine up to about 1/2 or 3/4 throttle. At this point and anywhere above it falls on its face. It seems to me to be starving for gas. I have changed the fuel filter, bypassed the aluminum block that has vacuum and fuel lines going to it (this appears to be either a fuel pump or a safety fuel shutoff), dissasembled the carb (except for the float bowl, which has 2 stripped screw heads), cleaned the carb and blew it out. I still am getting the same problem. After reading some similar problems in this forum, some are pointing to a plastic nut on the clutch and others are pointing to a flat lobe on the exhaust. This 500 also has somewhat of a knock at idle and floating at part throttle. It does experience some full throttle pop from the exhaust but it sounds and feels like a lean condition. The spark plug however reads light black and dry (normal). This is my best guess using a fairly extensive automotive and racing background. This is however my first experience with atv's. Please help with any input you may have. The 500 is fine for chasing cows or pleasure rides, but i would like to scare the bejezus out of my wife next time i take her for a ride. PS top speed is about 30 mph the way it is.......

 

I believe the block you are refering to is the fuel pump. It sound's like a fuel pump problem. Doesn't get enough gas at full throttle.
Check the diaphram in the fuel pump and also check the impulse line that runs to the carb. it may be cracked or unhooked.
Either way you need the pump or it will not run at all when the tank gets low.
Hope this helps.

 

Check your intake boots and it aint hat hard to check the valves they set at .006.