Not sure what you mean by springs back at idle. I don't know this bike at all but check for free movement in the cables, the grip, and at the carbs. Maybe disconnect the cables, check the grip, and lube cables. Then check and lube the linkage and butterflies on the carbs. I'm sure someone else should be able to help more than that.
08 Ktm 690 SMC
03 Honda Shadow 750 ACE