You’re over-thinking it. It’s all there.  In fact, too much there.   That gasket doesn’t belong there.  The “needle” isn’t a needle, but just a screw that limits how far the throttle shutter closes at idle speed (ONLY).  That affects how much air gets through and thereby also the fuel ratio.

Just dink around with it for best idle, remembering it has no effect unless the shutter is closed against the adjustment screw.