Can you see if your throttle is open/closed by looking into the air intake? When it's blocked, the throttle is closed. When it's open the throttle is open (100% RPM). I assume you can tell by looking at it what is full throttle and zero throttle. Take the control rod out of the servo arm. With your hand push or pull the control rod to open the throttle all the way open. Loosen the screw that holds the throttle arm to the carb. Set the throttle arm so it swings as much behind the screw as it does in front of it when you go from zero to full throttle or visa versa. You may have to keep doing this until you have the throttle arm swing/throw perfectly in half around the screw. Now that you have the throw correct on the throttle arm, put the throttle on full, then reconnect the rod to the servo. If it was correct before it should be right now. If you have a computer radio this is much easier by setting the end points to the fully closed and fully open positions of the throttle. Then your trim sets the idle and engine cutoff point. You may have to experiment with different holes in the servo arm or throttle arm to get the proper throw need to stroke the throttle from closed to open etc.

With a computer radio, you should make sure that you use as much of the servo throw as possible to get the most accuracy out of you throttle control but still have some left over for idle trim and engine shutoff. Adjusting engine throttles is a b!tch, but you can master it if you keep at it.