Carburetion
Pre-1980 models use the twin 38 mm (1.5 in) constant velocity Mikuni carburetors that can be tuned by moving the needle clip position, or by replacing jets. In the carburetors the velocity of the fuel mixture through the venturi, regulated by the opening of the butterfly valves and engine speed, causes a pressure difference between the top and the bottom of the carburetor pistons. This pressure difference raises and lowers the carburetor slides, increasing or decreasing engine output accordingly.
Post-1979 models use smaller 34 mm (1.3 in) Mikuni CV carbs with needles that seem to be listed in parts menus as being 'fixed' position,(in other words a needle that may only have one clip position). The pilot and main jets can be changed for different sizes. If the 34 mm (1.3 in) CV carb needles only have one fixed clip position.
As previously noted, of '81-'83 models using; Hitachi carburetors with all jets pressed in place. That was not correct for the XS650s. That is true for the later model, '80-on, XS triples, Yamaha's three cylinder 750/850 model.
Read more about this topic: Yamaha XS 650, Design