2000 Sportman 500

Any ideas on this? Doesn't run well in warm weather. I believe it is running lean, but not sure what I can do. It wants to creep when it's in gear and if I set the brake it dies (I think that is the one-way clutch and/or clutch alignment). Sometimes when I hit the throttle it kinda backfires and surges backwards. I also have noticed that when in reverse it sputters and coughs unless I push the reverse override, doesn't matter if it's in 4 or 2 wheel drive. Also, when in 2 wheel drive and I press the override, the reverse light comes on. The thing that puzzles me, is why does it run better with the overide pushed? Any thoughts on this?