[Nasher]

Worth whipping the carb tops off, inspecting the diaphragms, but also seeing if the slides do indeed move in the bores smoothly.

A set of carbs I had arrive with a bike that hadn't been used for a couple of years looked OK but the slides had sticky oil/fuel residue on them which stopped the slides moving properly.
A quick clean had them working fine.

It's also worth noting that the springs might have been mucked about with.
For example the Dynojet ones are a little less powerful than the standard ones, which would give you the opposite issue, but there could be some spacers or something in there.
I've found some odd mods on old bikes.

A quick google will reveal the visual difference between Dynojet and standard springs so you can see exactly what you've got.
And whilst you are about it look at the different shape of Dynojet and standard needles.

So many of these bikes have been fitted with Dynojet kits it's possibly rare to find one which hasn't.