[RichardJ]

You say you have the mixture screw hanging on by a single thread. That puts you clear on the right side of this graph. rpm's will only be a little lower, but the engine will be blubbering and running rough.
You are looking for an adjustment where the rpm's will drop off very sharply, but be patient. 1/2 turn at a time.

Engine warm, choke off, dwell and timing taken care of.

The screw may end up being 2 1/2 turns OUT from seated or 1 1/2 turns OUT. IT DOESN'T MATTER, as long as it responds to your adjustment.

If it doesn't respond to your adjustments until the needle is fully seated or even not then, you have a problem.

Make sure you don't have any vacuum leaks. Spray carb cleaner around the intake manifold and around base of carb. rpm's will vary if you spray at a vacuum leak.