Sounds like it could be running very rich, and opening the choke just increases the throttle opening enough to keep it running. You could just try adjusting the mixture, although at 10mpg and with such bad problems I would suggest re-building or getting a replacement carb.
Last time I did mine I got the parts from a local carb specialist, I got gaskets, needles, jets and dampers. Still plenty of places selling SU parts, I'd try the Yellow Pages first, plus these places are usually cheaper than the Classic specialists.
Richard