Fuel Idle Mixture Screw with O-Ring - Fits: XS650E/SE/F/SF/2F(1978-79) Stock Mikuni BS38 CV Carbs. Beware! Carb idle mixture screw O-Rings included may or may not fit correctly in your Mikuni BS38 CV stock carb. The O-Ring in the carb as originally supplied by Mikuni/Yamaha varies in fit and size and we have found it impossible to match or guarantee a proper fit. No alternatives can or will be supplied. The mixture screw and matching O-ring was never sold or supplied by Yamaha or Mikuni.