Issue with 04 Rancher

Hi all, I need your advise. I just purchased a used 04 Rancher 350 4x4 manual. The bike has been sitting and left outside for awhile. I took it to a reputable bike shop for servicing. The brakes were frozen up and have been fixed, yet they don't seem to respond quick enough compared to other quads I've driven. The shop changed all of the fluids (possibly not brake fluid, need to check). They said the bike had some water in the differential but they flushed it good and filled with quality oil. They also did a tune up.

I drove it tonight a little and noticed that the throttle kind of hesitates at first. Also, it back-fired on me tonight.

The shop thought the carb was good. Do you think it needs more servicing or to be run for awhile and see if it "clears" up?

thanks! Tapp