Newbie w/ 02 350 rancher fuel sys. prob.

I am looking for some help. I bought an 02 350 rancher tm2 with low, low hours. It has been sitting for over 3 years. I put a new battery in it yesterday and treated the gas with the seafoam according to directions. It cranks right up, but I can't get it to idle properly. I have adjusted the throttle stop adjustment at the carb. w/o good results. Should I have to rebuild the carb. to get it to run smooth? It backfires when I downshift w/o braking. Thanks for the help.