A lot of engines (if not most) have the low speed needle right up there near the prop arc. Not a good place to be putting fingers while the engine is running.

The OS AX series of engines have an angled high speed needle, angled back with a length of very stiff wire to go into a hole with a set screw in the needle valve knurled body, and a little knurled thumb dial that mounts on the wire. This puts the high speed needle adjuster back further and away from the prop arc making that a safe and secure method of doing the high speed mixture adjustment.

CGr