99 polaris xplorer backfiring thru exhaust will start after a million pulls and will idle for a few seconds then bam

 

Could be a few different things. Check the carb boots for leaks. And it sounds like you now have an exhaust leak. May want to do a compression check just to see where you stand on that. Below 90psi and you'll need to rebuild the top end. If so, you'll have the carb off then anyway, and might as well rebuild it too.

 

is this the 300 or 400 xplorer?