BSAs need a thermal insulator between the carb and manifold. These are NOT
an Amal part, and therefore DO NOT come with new carbs or the Amal rebuild kit. However, not all thermal insulators are equivalent. Some won't seal, some are too thin. Problems abound which usually mandate a fuel-proof sealer, AND a very calibrated way of fastening the carb... lest the mechanic use too much torque and distort the carb body... causing more air leaks.
All things considered, the BEST
modern solution may be
the Triumph carb mounting solution. This design uses a fat o-ring to seal the carb to the manifold. The o-ring is so big in cross-section that the carb does not physically touch the manifold, providing thermal insulation and a BETTER seal (without using sealers). Then, smaller rubber cushions go between the mounting nuts and the carb body on the air filter side of the flange. In effect the carb is now rubber mounted at 3 points, with a 100% seal to the manifold and WITHOUT distorting the carb body.
You can see the effect in this photo of a Bonneville....
Again, these 3 new "rubber" parts are NOT Amal parts. They are purchased separately, but they need not be sourced from Triumph. O-rings good enough for this job can be sourced at any local hardware or plumbing store.
Hope this helps.