Remove the linkage from the carb slide, move the slide in and out, it should be very easy almost silky smooth. The rubber bands should not be used as a return spring, they are only there to act as a fail safe. The spring on your throttle linkage is what returns the slide to the closed or any intermediate position. The collar that holds the spring against the servo horn should be adjusted so there is a slight pressure to alleviate any slop in the link. The idea is to allow the servo to reverse during the use of brakes without limiting the brake travel.