K301 Backfires and more

Hi all, I have a K301 12hp that I bought 4 years. The guy I got it from said it was a shortblock from Kohler. It ran great for 3 years until it threw a rod. I took it apart and found that when the conecting rod broke it also broke off one of the wieghts on the cam. I found a used cam with the exact numbers on it and I got a used crank and connecting rod. I put it back together and it started right up but backfires when going from full throttle to idle. I put a new carb on but no change. I also noticed now at full throttle it looks like a steady flame comming out of the muffler. This is my first time working on a small engine. I do have the manual for K series engines. Do I have somethine wrong?

Robert