An external idle air bypass for carbureted engines having a threaded housing containing a ball valve, which mounts to the outside of the carburetor at a passage which admits air to the intake plenum beneath the carburetor's throttle blades. The threaded housing may be made of metal or other materials. The ball valve may be made from brass and steel, though other materials can be used. A handle is used to operate the ball valve and is retained by a threaded nut. An air cleaner may be attached to the valve housing. Opening and closing the ball valve permits a precisely metered amount of air to enter the engine at idle, enabling the mechanic to vary the amount and air/fuel ratio of the engine without disturbing the throttle blades.