What causes backfiring in Kawasaki 220 bayou?

I have a problem. I just bought a used 1995 Kawasaki 220 Bayou. The engine started running a little rough at first. Kept getting worse and then started backfiring through the exhaust. I had the carbeurator rebuilt but still runs rough and backfires. Cranks good and idles good. What could be my problem? Thanks