My mech told me you've got to do it with a vacuum gauge and its a real bitch taking the carbs off/on many times to get it right.
As I understand it, balancing is only to get smooth power at partial throttle openings. At WOT balancing is not an issue.

I have only just put my bike back on the road after it lived happily in my mates shed for 10 years, the carbs were all over the place hence the garage, I stripped them completely and followed the thread, kept to the .5mm washers.
I then checked the butterfly mechanism to find that they were all different(some open some closed), it's worth spending a little more time looking at what it should be doing rather than what you think it should be doing if you know what I mean? The choke mechanism was also coCked up, they all opened in a sequence rather than synchronised. After adjusting them all back to where they should be I installed and fired her up and the garage near me told me not to bother ballancing the carbs because they are spot on.
However after a while it started to show signs of lag like you mentioned and after another strip down found pipes where kinked and split , make sure you give it a thoughrough check over and try again.

Looks like it's coming apart again then. I'll take the carbs apart and use the 0.5mm washers, fit them back up again and check everything is in sync manually.

I've spent the afternoon taking the carbs off, stripping,re checking and making sure the buttterflies were in sync. I've changed the washers to 0.5mm, swapped the jets from 120 front and 122 rear to 118 front and 120 rear, and still the bike revs really flat at the bottom until it decides to burst into life about 3500-4000 rpm. My heads really battered now. Anybody got any more suggestions as to what else it could be?

Right.

Lets get some facts before we make assumptions.

Have you ridden the bike yet or is this just revving the throttle?
Have you let it warm up, there not much good when cold?
Do you have the standard airfilter / airbox in place.

Have you had a look at the plugs to give you an indication of how it's fuelling?

Balancing the carbs is easy, but it'll only give you a smoother idle. You just need a set of vaccumn gagues (like the morgan carbtune) and preferrably a right angle screwdriver.

Attach the hoses to the vaccumn ports on the cylinder heads, set your idle and turn the 3 adjustable screws until the bars line up (one carb is fixed)

I've not checked the colour of the plugs yet but will do tomorrow, i've set the screws to 2.25 turns out. The bike is running a standard filter and airbox. I've also run the bike up to temperature before hand.

Just a thought, but if the inlet rubbers had hardened up with age and weren't sealing properly would an air leak cause the problem i'm having?

Air leaks will make it lean.

Signs of an air-leak.

Bike wont' hold a steady idle

Revs will hang-up before comming back down.

Since the bikes all together and running.

Start it up and let it warm up a little.

Set the idle to 1000rpm.

Crack the throttle

Does it bog before it picks up?

When it comes back down does it hang before comming down.. espeically around 2000 or does it return to idle?

or does it drop lower than idle then pick back up again.

The revs do hang a bit when the throttles shut and wont idle at less than around 12-1300 rpm even when warmed up off choke. The bike bogs when the throttle is cracked open as you say and slowly returns to idle speed, doesnt drop below and pick back up again. Would the bike running lean make the plugs really clean looking as when i took the rear 2 out they look like brand new. They are brand new but i'd expect to see something on them by now and theres no deposit of any kind.

If you need any help, drop me a PM and I can bob down and give you a hand. A fresh pair of eyes might spot something you may have missed