B33 Carburettor

[Mark B]

First thing: it's a 1954(ish) B31, engine number BB31 18580, which has a B33 top half. How do I confirm that it actually is the 500 top half?  The carburettor inlet is 1 5/32 inches. 

The carb is stamped on the flange 376/4 2 and I can't find mention of this specific one (the /4 2 suffix) in the Amal catalogue.  Have I got the correct carb for the engine? It's being ultrasonically cleaned at the moment, and I want to check the jetting etc when I get it back.  Also, any suggestions on jet sizes etc - it's got a gauze bellmouth on.

Thanks for any advice, Mark. 

[Pete C]

The 376 monoblocs should be 1 1/16 bore and the earlier 286 pre-monoblocs 1 1/8 for a B33. So the inlet tract on your head should be somewhere around those values rather than the 1 inch which is standard for a B31 head. That's assuming nobody has done any work on the tract in the last 65 years.

The inlet valves are also different but you will need to at least remove the head to measure that (I think).

I once fitted a gauze inlet bellmouth to my B31 and chucked it in the hedgerow an hour later. It completely screwed up the performance and I couldn't be arsed to fiddle about with jetting on what was a great runner on an open inlet and standard specs. You may have more patience than me!

cheers
Pete

[Richard Mills]

I put an Amal gauze bellmouth on my 1958 B33 and did not detect any change in performance.

[Mark B]

Postscript: I've just spoken to a very knowledgeable person at Burlen/Amal who explained that my bike would originally have had a pre-Monobloc, and the one I've got is originally from a Triumph Thunderbird export model.  The jetting seems correct - 25 pilot, 106 needle and 260 main, so I'll keep it as is.