[ROVERNET - UK] Ben's SU problem

[Fletcher]

Ben-
This may be key: 

"jet needle had a small ring groove around it, it was
approx 1/8th inch out from base of piston"

This is a result of corrosion at the surface of the fuel in the jet, caused by an interaction between moisture in the air and stuff in the fuel, especially bad on cars that sat with 1980's to early 90s fuel. And even worse with some aftermarket needles, especially steel ones. Check with a magnifying glass and you will see pits making up the "groove". Since the clearance here is on the order of .001" in the idle/slow run position, any pitting makes it way too rich. You try to compensate by leaning it out, which might work, but it makes the entire range above slow run very lean, giving the symptoms you describe. The previous setting with the needle too far out of the piston was likely an attempt to work around this, but would also have resulted in too lean at idle, then very rich just above idle, then lean again above that. New needle time!
I would be very interested in seeing a diagram of the "economiser" device - never met one, must be a Rover only item, it is not on any other HD carb I ever met. Some much later SUs had something similar, but again I've never seen one, since they came on Minis and such after we were deprived of these.
FRM