My 2005 Sportsman 500 will start and idle fine. But then when the throttle is applied, and held, it immediately chokes and dies. If I just tap the throttle, it revs then returns to idle. Only when I hold the throttle, even just barely, it dies. I used some gas line antifreeze, but it didn't help. Then I took apart and cleaned the carb, still no help. Any ideas?