I was asked to work on a 2006 600 rhino for a friend who just bought this thing from some family member. He was told they took it to a guy to have it tuned and they were never happy with it again, hence why my friend now owns it. When cold it takes a long time to crank it even with choke on. If you give it a quick shot of start fluid it will fire up and run. Idles is rough until warm. Once cranked and warm it will crank better but still not like it should. Does have power and runs great with the exception of one large pop/backfire every time you come down from WOT. This machine looks to have aftermarket exhaust with no silencers (look like regular exhaust tips), and what looks like a K&N filter with stock air box. Carb has a 158 main jet and A/F screw is sitting at 4 turns out. Needle is currently sitting on the #2 slot (the middle). I have tried moving the needle up and down and changing the A/F needle to 2.5 turns and then slowly up and nothing has seamed to make a difference. If anything made it worse. When cold if you try to give it gas it just back fire through the intake and dies. He has a jet kit that I can use to change jets around if needed but I need some advise on where I should start. Any and all help is greatly needed and appreciated!