Why does my atv backfire and sputter when i try to start it?

I have a 2000 polaris scrambler and when i go to start it; it just backfires and sputters; every once in a while it will start and run just fine but as soon as you turn it off your back in the same boat as before. Ive changed the plug and wire; and that doesnt seem to help. Could it possibly be a bad switch on the handle bars?