Hi guys, I've got a low-hour 2004 CRF250R, low enough that the stock valves are still in spec. I've recently passed the bike down to my son and he started commenting on how much the bike was backfiring on decel. Started listening and, sure enough, the thing is backfiring like crazy with the throttle off. I re-jetted this thing a few years ago, and it was working fine, however, the bike has been sitting for over a year (with gas still in it). Where do I need to start my troubleshooting? Thanks!