They put that plastic gear in there so you don't burn the motor up (gear strips before servo motor goes). I would have to agree with the above posts. Something is binding it up, either your linkage or maybe one of your medal gears inside the servo is bound. Does the servo work OK when the linkage is off? I had a servo go on my Mugen and found out that my throttle return spring was putting to much pressure on the servo when at neutral. I adjusted it so it barley puts any pressure on the servo, then use a rubber band around the carb slide so the carb goes to full close when the servo is at neutral.