Found the problem
I thought I'd try raising the needle just to see if it had any effect (playing with the choke is not easy when riding around where I live). Anyway, when I went to remove it I found that the nuts on the manifold studs were somewhat loose, so I tightened them up and gave it a try. A big difference!
Of course one is warned against over tightening them and warping the manifold flange on the carb!