Unscrew the two screws from the fast idle valve, shown circled in the image above, and remove the metal lid. [I used a #1 Philips head.]

Inside you'll see a beige plastic valve with a spring assembly running through it. This plastic valve is threaded and must be securely tightened in order for the motor to idle correctly. In my case, the plastic valve had completely unthreaded itself, and was sitting loose. There are ridges along the side of the valve into which a flat-blade screwdriver can be used to tighten it.