Original: In many diagrams for emissions-controlled models, the mixture screw is missing entirely. A "cut here" dashed line over a brass plug on the underside of the carb, with an arrow pointing to the screw underneath. This is the most common "patch" added by home mechanics.
Here are diagrams and exploded views of Mikuni BS and similar series carburetors to help identify these parts: 15+ Mikuni Motorcycle Carburetor Diagram Carburador Mikuni BS25 | PDF Sudco Intl. Corp. - Mikuni Diagrams Mikuni Carburetor Tuning, by Victory Library Victory Library mikuni bs25 carburetor diagram patched
How to adjust the air mixture screw on a Mikuni carb with the choke down? mikuni bs25 carburetor diagram patched