i have measured the 4 throttle baby vacuums and compared and I find that #2 is wY higher than the rest and seem to be unable to adjust screws to sync, I have checked with a spray of engine start and also propane, check the rubbber as best I can.
Any ideas please 
I have also just complete a valve clearance check and all are ok

The passage the air screw controls is probably blocked.  Try removing the screw and shooting carb cleaner in there.

or a pipe cleaner soaked in carb cleaner, through them all

but the feed passage to the valve could be blocked as .75 suggests