If by sputtering you mean it runs on for a few rpms after ignition is turned off it means your primary throttle plate isn't closing all the way. Could be your return spring isn't pulling enough (set too short, not strong enough, whatever), or the linkage is binding, throttle plate is hanging up if gasket is wrong, or idle linkage screw set too high. Check everything that affects the linkage:wires, air cleaner, and so on.