As I have said a million times before, the purpose of the spring is not to pull the servo back to idle. Let me ask you this. If your throttle lingage becomes disconnected from you carb arm, what good is the spring going to do if it's attached to the servo arm? It isn't going to do a thing. Now if it's attached to the carb arm and the servo arm comes off, breaks, or any other part of the throttle linkage becomes disconnected, the spring will do its job by pulling the throttle back to idle. The only proper way to move the servo to the idle position is with the use of a fail safe. Yes, the servo is always going to have to overcome the spring, but the spring should never be strong enough to override the servo.