The 276 is the correct carb up to 1954 when the Monobloc superceded it...(We only tend to think War Department here!).
The Monobloc and the Concentric were an improvement over the 276 in that they are less prone to fuel surge/starvation due to the differing positions of the float chamber, have less joints and there were some detailed refinements to the fuel delivery...
In essence they are very similar though and the differences when related to the performance of an M20 can pretty much be ignored.
Tuning procedures are the same for all types apart from very minor differences like the number of needle positions available...
The Monobloc can foul the dynamo so a larger spacer and longer carb studs are required for it...I'm not sure about the Concentric...I've never fitted one to an M20.
The one you have is too large at a 30mm choke size....This is of no benefit. A 26mm concentric would be the more correct fit...
For jetting a Concentric use the same choke size, jets/settings etc. as those used for an M20 that had a Monobloc fitted as standard (54 0nwards) and then 'fine tune' once the bike is running...
Bear in mind that Amal carb specifications for a particular model are not 'cast in stone'....
Due to production tolerances in both the carb and the engine,variations in fuel and varying degrees of wear, they should be regarded as a 'base' setting to get the bike running.
You may, or may not, have to make small adjustments beyond these base settings to get a good result....
Finally if you are considering a new carb and aren't worried about originality the Concentric is by far the cheapest of the three types....
Wassells make a replica of the concentric that can be bought through many British bike dealers (They have 'Wassells' cast into the body)....Take it from me, whatever you might hear to the contrary these carbs work very well and are the most cost effective new carb out there....You should get one for about £80 or so...
Running weak can be bad for your engine so when setting up, particularly if you are not familiar with the procedures, acquire a manual that describes the procedure and set up 'rich' initially, weakening the settings progressively to get a correct mixture....Ian
email (option): ian@wright52.plus.com
Oops..
...I forgot Richard had a later bike with a WD engine...So your quite correct Ron...Ian
email (option): ian@wright52.plus.com