Check and make sure the carbs are opeing and shutting properly or that they arent stuck open or closed. I was working on a buddy's banshee and it wouldnt start unless we sprayed starter into the air intake and it was hard as hell to start and would idle really high and then die. Turned out the right carb was stuck open. It was really easy to fix, we just took off the top of the carb and reseated the piston inside.
Hope it helps.