On the old O.S. FS-90 engine, there is no idle "needle". The idle mixture is controlled by a shaped port in the spraybar that gets opened and closed by a tube that slides over it. To get the idle mixture adjusted to a rough starting point, you must first open the throttle to full. Use a small screwdriver to adjust the idle mixture screw that's in the center of the throttle arm. Turning it clockwise will lean the mixture, turning it counter-clockwise will richen the mixture. Look into the venturi when the throttle is fully-opened. Turn the idle mixture screw clockwise until you see a shoulder start to protrude from the side of the carburetor barrel. This is on the same side as the mixture screw. When you see the shoulder emerge from the side of the venturi, turn the screw counter-clockwise until it is flush with the side of the venturi. This is the starting point. From there, you will adjust the mixture as needed, rich or lean, to get the desired idle and transition performance you desire.