If you are buying a new carb direct from amal, take care , it took 4 months to get mine. as it turned out they did not have one made up and no orders, so kept putting me off. Terrible customer service. They even charged me postage on a cable adjuster, which they forgot to include in original order. Then it had to go back as bike ran rough, like yours . Turns out they had fitted 2 stroke internals. Mines a 1967 B40, all sorted now and runs lovely.
Best service is from surrey cycles 01342 716 120.
If I had known at the time could have had one next day, lovely people