buy a throttle return spring from AE and you unscrew one of the socket head screws that holds the engine to the mount and then you put it through the spring holder and screw it back in, then you Atatch one end of the spring to the spring holder and the other end goes on the throttle arm. The AE spring isnt strong enough to pull the servo when the battery is dead so if You want something like this you will have to buy a stiffer spring.