RZ250 won't pull under load.

[jools]

Here's one of those silly ones. Been there before, but this one is annoying me.
'83 RZ250. Very original unbodged bike. 1st oversize, all that done recently. new PV stuff, carb cleanout and float valves etc, all the obvious things checked. (mind you I've found mismatched plugs).

The thing just won't pull low down and under load. It's like a vicious powerband, Feels like it's going to die, then once it gets up a bit it pulls like a teenager, but it just totally bogs down at low rpm if there's any load on it.
PV adjustment / travel looks good. I've by-passed the fuel tap vacuum system -running on PRI, this has made no difference, air cleaner all good. Plugs look very oily but the thing hasn't had a chance to get hot and clean itself out.
I know it's going to be something simple and I should have picked it, but any ideas? Yes, been freshly rebored, rings, run in. It's not something that has happened because of work done on it. This symptom has just occurred out of the blue, it had been running fine.

[2TInstitute]

had worked on 2 variations of this on RZ's recently, first 1 cylinder really lean and one really rich conclusion was a air leak between the cylinders but never found, and another that wouldn't rev over 6000, that turned out to be electrical.
Sooooo given your rich maybe an air corrector blocked or intermittent spark??? Which real odd because I don't remember RZ's ever having issues like this back in the day

[jools]

Well, it turned out to be one carb slide had dropped of it's cable. Horrible little spring clips on the slides lost their tension, so I just re-tensioned them. But I wouldn't have expected my symptoms with one slide not moving, or perhaps somehow a venturi effect pulled it up once the other cylinder was 'on the pipe'.
Easy fix (although wrestling those carbs on and off and re-connecting everything is no fun!)