its just a spring: http://www3.towerhobbies.com/cgi-bin...&I=LXDTA1&P=ML

you can use one from a full sized carb for a car or for a screen door, home depot has them

one end goes on the throttle arm, the other end wraps around the back of the engine and hooks to something (screw, bracket) so if the throttle servo goes dead, it can pull the throttle closed.