Do you know anyone that might have a carb that they would lend you just to see if the problem remains with the loaner? How about pulling plug wires one at a time at idle to see if you have a dead or weak cyl at idle?

You need to determine whether the problem is the carb or the engine itself before doing anything else.

One last thing, do not worry about any particular A/F ratio at idle. I see good idles anywhere from 13.5 to 14.5. The idle ratio doesn't matter, just tune for what the engine likes. Of course, if the idle mix is way off, then the O2 can tell you which way to go. If it has a mild cam, then a vacuum gage works good or you can just tune it using a good low RPM tach.