Ok, here is the issue. The throttle arm on the carburetor is adjusted differently than the one on your stock motor.

Remove the carburetor. See the arrow in the picture I posted? Its pointing at a nut. Loosening that nut will allow you to adjust the arm. If you still have the stock carburetor, adjust its idle screw so there is a 1mm gap. Adjust your new carburetor so it also has a 1mm gap. Move both carburetors to their idle position. Now adjust the arm on your new carburetor so it matches the old one. Install it back into the car. Now remove the servo horn from the servo, power on your radio and receiver, move the idle trim to neutral. Put the servo horn back on the servo making sure the carburetor is as close to its idle position as possible. Adjust the throttle trim on your transmitter to take care of any small adjustments that may be required.

If you dont have the stock carburetor, then it will just take a bit more trial and error adjusting that arm to the proper position.