Is my Vacuum too low?

what he said
but at 750 rpm and 12* initial, IMO,it should be higher. Most STOCK teeners I have worked on are up at 18", at much lower rpm and waaay less timing.
So if it idles nice, I suspect a bad gauge.
The vacuum depends a lot on the ring and valve seal, so you might want to do a compression test.

The mixture screws are usually out about 2.5 turns, but that depends on how far up the transfer slots that the throttle valves are. I suspect the 750 rpm has something to do with it.
So my question is; why 750?
At 12* initial, I would think she would idle well at 650. That 100 rpm alone, could account for the number.

But with the mixture screws almost falling out, that's messed up. I suspect a problem in the low-speed circuit, and your solution was to crank up the rpm.