Paul sounds like a carb problem right enough. However also check ignition timing, HT leads, correct points gap and tappets are all correct (crucial!). After which thoroughly clean out the carb paying attention to float bowl, make sure no grit or grime is present. Check fuel is free flowing into carb and float is fine. Check throttle needle is not worn, fix height at manufacturers setting (half way?) Make sure bottom flange nut is screwed up tight and all washers and O rings are present on the carb. Popping through the silencer is a fair indication of a weak mixture – plugs should be slightly brown in the centre to pale on the outside, not whiteish. Verify there are no air leaks at the flange gasket (use gasket cement as fail-safe) and the throttle slide is not worn (look for scratches in the upper bore?) – also check that exhaust pipes are sealed at the cylinder head or are a good tight fit, air getting in here can also cause popping? Test the bike with a run at near full throttle, if the engine runs heavily it is running rich and needs a smaller main jet… if it seems to pull better as the throttle is slowly rolled off then it is running weak – check spark plug colours to verify. Test tick-over by screwing up throttle stop screws to fast idle then drop throttle screw until engine just begins to falter, then adjust the air screw back and forward until you find the best position that makes the tick-over faster. Then readjust throttle screw for desired idle speed. Suggest you have air screw at 1.5 to 2 full turns out from fully closed to start test and do fit new O rings to both screws.
If problem persists after all checks are carried out then I’d suggest the only cure is a new carb but well worth the investment. I had the same problems with twin Amals on my BSA Lightning – new carbs sorted it after all the above tests didn’t help. It would run perfectly OK but would cut out on idle when warm. The throttle slides were worn. Good luck!
