I bought a used 2004 sportsman 90 for my kid a couple years back , it ran fine for awhile and then it started acting up by starting hard, not wanting to idle, and then it would idle but would bog down on acceleration and die.. after doing the simple stuff like spark plug, air filter, carb cleaning, my neighbor who was a motorcycle mechanic in his younger days diagnosed a low compression situation on the motor. I ordered all the parts to go thru the motor and hired him to do it. The starting situation improved, but we couldn't get the carb adjusted make it run right from idle to acceleration. My neighbor researched these small ATV's and saw that the carburetors they sold said they were good for both the 50 and 90cc ATV's. In his experience he said that the jetting is going to be alot different when you almost double the size of the motor. I ordered both a pilot and main jet kit of of Ebay and my neighbor talked to some of his fellow motor heads and got some preliminary jet settings to start with. It took about 4 attempts, but we found the jet sizing that made the machine run beautifully, starting easily & accelerating smoothly with nice top end performance. For those of you who've beat their heads against the wall to make their sportsman 90 run right, try an initial pilot jet size of 32.5 and a main jet of 95 and set the pilot adjustment screw out 1 1/4 turns. Drive the machine a bit and get it warmed up and fine tune the pilot adjustment screw and idle speed adjustment on the carb slider.