[Darren69]

It sounds like your shims are pretty much in spec and I doubt if that would be the problem you describe, if just a bit out it should still run ok. I agree that you are right in saying that they run better when its just spot on, just after a service you usually can tell the difference. However your logic appears to be correct and the problem does appear to be idle jet/circuit/mixture. So did you balance the carbs? I'm wondering if its that because I can't think of anything else atm?