MJ is correct and the small screw on the right is called a guide screw or guide pin. This screw is not adjustable and has a pin on the end that protrudes into the rotating barrel and runs in the concentric slot cut into the barrel. The action of this pin in the groove when the barrel is rotated causes the barrel to move inward and outward, causing automatic enrichment when the throttle is opened.

Guide screws, throttle stop screws and bleeder screws are often confused on various engines simply because on some, the function of the stop screw and guide screw are combined into one screw and still on others the stop screw has been eliminated altogether.

John