If what you mean by this is the throttle barrel does not completely close then you need to confirm that whatever your engine is, that if there is a throttle stop screw on the carburator, it is not stopping the barrel from closing completely. In other words without any pushrods connected to the carbs throttle arm that when you move the arm to closed position and look down the hole (Venturi) the barrel is completely closed. Many engines come with throttle stop screws that the fellows often ignore this.

John