Erratic idle speed

[evilgidget]

The Wifey's 33 1.5 twincarb has (yet another) problem that's been irritating me. After driving a little while & pulling up (ie at traffic lights), the car will slowly pickup idle speed until it's idling at about 2500rpm. Not good when you're parked next to a ricer as it gives them ideas (and me less of a chance ). Has an accompanying vibration, maybe as though one bank of cylinders is doing all the work. I think it's carburettor related, but I'd like some way of finding which one needs the repair.
Doesn't seem to have any sort of pattern as to when it does it, though it does seem to be doing it on a more regular basis.

Thanks