[Nickj]

So the carbs are mostly rebuilt, and the float levels are set and good?
Check the carbs balance, can be a pain as the motor revs slowly and there are a lot of big pulses of pressure. I use one of those ball bearing in a tube system which works OK.
The CV diaphrams ought to be rechecked for and pin holes.

Does the smokey exhaust smell oily or fuelly? the Mk I nose is often a good guide to a rich mix or oil burning. With compression within specs you'd maybe lean towards the valve seals.

If it looks like oil it may well be, you should have had really a really sooty looking plug IF it's coming out of the motor unless it's just leaking from the valve seal and not dripping out of the exhaust valve seal.

Also check the valve opener and closer tolerances while you're at it. The 600's don't make massive power to start with so any loss (or gain) can be noticeable.