Finally, success!

I took an identical carburetor from another engine.
I reset the pressure relief valve to original setting.
It runs like a charm.

I compared the two carbs visually. The original carb which I removed, with the low speed needle turned all the way in (so I thought), left a gap of about 1/16 inch from the tip of the low speed needle to the "whatchamagigy", the brass tube which extends into the throat of the carb from the side of the high speed needle. In contrast, the carb from the other engine (which had run fine), showed the tip of the low speed needle entering the brass tube, both of these with the carb "wide open".

So I checked to see if the low speed needle in the troublesome carb was in fact turned all the way in. I found that it was not....with sufficient persuasion (brute force) it could be turned in an additional 1 1/2 turns. Now the tip of the low speed needle is about flush with the end of the brass tube.

Either I have cut new threads or this was one tight fitting low speed needle. I shall now switch the carbs and see what happens.

The high speed needle is out about 2 3/4 turns, so I judge from the above comments that the pressure is in the ballpark.

The high end is 8,600 vs. 8,800, but it's only 55 degrees.

How say ye men of the jury?