Yup, I have a couple of those gasket sets for just a couple of them too! 
It is nice just calming down and going through something properly rather than patching all the time….

UPDATE RE GASKET SEALANT - AIR LEAKS ON CARB 1

ok so I gave in and once more, this time with patience, at idle, sprayed Easy Start on the front carb inlet manifold, several times, each time waiting for the engine to slow down, squirted and it went faster.

We have new Cometic gaskets and only the rear one has additional sealant round it. This was an experiment which now bears fruit and demonstrates that the extra sealant was a good idea. 

I found this was the case when fixing the coolant system and dealing with metal surfaces that are very old and mottled. In the case of the coolant system I replaced with new parts. It seems with these inlet manifolds the sealant is effective. 

So next task is remove carb 1 (again), remove and refit inlet manifold 1 front this time with sealant.

The clue to all this is the Manometer shows a MASSIVE difference between carb 1 and carb 2, carb 1 showing it has vastly more air getting in somewhere

Have you checked for flatness of the mating surfaces (with a straight edge) Anthony?  I have never found the need to use sealants on an inlet manifold (although never had a BDR)

Ah well, I did one with sealant and one without to see what happens. Now I know. The gasket was sopping wet on both sides, pic 3535 below.
Also below shiny one 3545 where I did the scrapey thing 7wow suggested, but tbh I have found with this age all the surfaces are suspect even when they look ok. It wasn't until I accepted this idea that I got the cooling system to work again after 20 years not really doing so.
The rest of the pics just show my progression through application, it squidges everywhere inside and out and then I slowly clean it all up with an acetone soaked cloth. I find the RED Motorsil D difficult to apply,

https://arexons.it/en/products/car-care/professional-car-care-and-maintenance/guarnizioni-liquide/motorsil-d/

it doesn't squeeze on from the tube and requires my finger to splurge it around. BUT it says its easy to clean off and indeed it is.

all sopping wet - so air leaks

after WD40 on 600 wet and dry

one side sealed

both sides sealed

cleaned a bit, it likes some time to part cure

inlet manifold fitted, no torque setting, just tight not F tight, because no tools can be fitted to the reverse side nuts except a very small 11mm open spanner.

and checking for sealant inside - more to do

underside to clean excess

topside to clean excess

more checking

more to do

cleaned the rest and left to cure overnight

Right then. All put back together, dizzy cap included (it's far too easy to forget that, I need some sort of visual reminder. Maybe just to disconnect the coil's HT lead at the coil end; or perhaps better the coil ignition wire so nothing can spin.

Well, Gordon Bennet, it was like going back in time a week or two; impossible to start, lots of backfires and so on. Gave up and tweaked the timing with the dizzy a bit, got it started (this is long frustrating story very short), strobe light, reset the dizzy timing and running nicely, reduced tick-over, balanced Carbs using Manometer, no need to tweak mixtures or air compensations because both pairs of barrels practically identical, to my pleasant surprise.

Anyway, the massive air leak problem has clearly gone. Forgot to check the accelerator jets, but that can wait; I have turned over all the pump jet "weights" all of which have ball bearing impressions worn in to them. TRICK: blow compressed air down the weight's hole and the weight jumps out - and hard to catch. 

Raining again so further tuning has to wait, but bit of a celebration at this balanced point.

oops

Dribbling : NONE AT ALL! 

So it was the ball bearings wrongly placed.

And MAYBE the weights having been worn down at the ball bearing end (weights all now the other way up).