few questions before we can give you a good solid answer:

1) does the carb just stop moving? in other words does it seem like the servo tries pulling the carb closed, but it just stops and binds like the carb wont close any further? if so, look on the carb for the idle stop screw. on most engines there is a groove in the carb barrel that rides along the stop screw. the hight of the screw in that groove dictates how far the barrel will turn. loosten it.

2) do you have an ATV (adjustable travel volume) function on your transmitter.. and is it set to decrease throw on your throttle servo? if so, make sure its turned off.

3) are you using a soft throttle cable? check for kinks, bending, or binding. also make sure its not too short or too long.

4) is your servo arm centered on the servo shaft? take off the arm set it so that when the throttle stick, servo, and carb are in the middle position, the servo arm is close to the center of its travel.