Have just finished doing the valve clearances and would like to balance the carbs before putting them on. Can anyone tell me how this is done as in what size drill bit ? Where does it go exactly ? Is there a base carb ?
Any tips on doing this would be much appreciated.

Looking into carb No 2 from the port side, 3 tiny holes can be seen with the butterfly opened.
Close the butterfly and adjust the throttle stop screw so that the edge of the first of the 3 holes is only JUST visible.
Adjust the other 3 with the balance screws so that they look the same.

To check, pack the throttle stop with feeler gauges so that a plastic straw [from a WD400 or similar] will slide between the carb body and the butterfly.
Adjust the throttle stop screw so that the straw [or similar size drill] will just slide in and out of No2 carb with a light pressure. Check, and adjust the other 3 if needed, but the initial setting using the holes will be close.

You can set the carbs up with vacuum gauges when fitted to the engine, but if the tickover is even and the revs do not fluctuate, [look at the rev counter] then leave it that.

Great, thanks for that,will give it a lash. So number 2 is the base carb,yes?

Correct

Just had a look there just now and i can set them all up so the edge of the first hole is slightly visible in all four carbs. That's fairly straightforward . Should a 1mm drill bit fit down between the carb body and butterfly now? In order for me to do this i would have to open up the butterfly way more and all three holes would be visible.
Think i got it wrong again dad.

Alright i just reread the procedure and i have the same feel with the straw on them all after packing out the throttle stop with the feeler gauges. Thanks again.

Look at my previous post again.
A 1mm drill will not fit in a minute gap unless the butterfly is opened.

Now the holes are lined up, the carbs are near balanced, but if you open the butterfly by sliding a pack of feeler gauges under the throttle stop so that the straw or drill is a sliding fit in no2 carb by adjusting the throttle stop screw. [ Easier than winding the screw all the way in, it may not lift enough anyway] they can be double checked in the partial open position as well as the initial almost closed position.

Adjust the 3 screw if needed, but balance will be close already.

Composing whilst previous reply was posted.

All good. Thanks for the input.