I have a feeler gauge set that I have modified 3 of the blades (19,20,21) to be able to fit into the throat of the OS.91 carb. The three blades add up to .060 or 60 thousanths. Put the gauge into the throat with the throttle opened to full and adjust the low speed needle to this clearance. Once set, I have never had to go back and change the setting.