BS--The big plus is compensation for altitude and other atmospheric variables. The minuses are limited tunability for performance mods, less than optimal performance unless a still air box is used, and somewhat softer throttle response than carbs with mechanically operated slides.
VM--Pluses are low price, enhanced performance (assuming the rest of the motor will deliver the goods), tunability, mechanical simplicity, availability of parts, and a deep well of XS650-specific data to draw from. The downside includes touchy synch adjustment; idle synch is adjusted by the same throttle stop screws that control idle speed, and lift synch must be set separately--and often, since uneven cable stretch will disrupt it. The design is old and less than optimal from a performance standpoint, but it's a lot of bang for the buck.
CR--Sorry mah man, that's a Keihin. Pluses are many. Keihin quality is high. Unlike the Mikuni VM's, the CR's are dedicated 4-stroke carburetors. Slides are lifted by throttle shafts, allowing for separate idle speed and synch adjustments, less frequent readjustment of synch, and only one adjustment for synch. Minuses are price (on the high side) and need for fabrication; carbs must be bracketed, and a throttle shaft must be fabricated for the XS650's 120 center-to-center intake distance. The data pool for the XS650 is near nil; for those who know how to tune this shouldn't be a problem, rookies will wish they'd kept their cotton pickers off these. Like Mikuni VM's the design is older and doesn't represent the state of the art, but the CR's are a good deal at the price.
