when i first bought my old 55 with the engine in it, it too was popping through the carbs and out the exhaust, so i gave it a major service, still doing it? off to the carby place to get the twins tuned, and the told me it was low on compression, so off to the mechanics and found out i had bent valves, thats what was causing it to "pop", when it tried to fire on the compression stroke, since it had bent valves it didnt have a good seal and when it fired it escaped back through the carbs and out the exhaust, this was caused through the camshafts not beiong timed properly and the valves had been hitting on the top of the pistons, i really hope this isnt the problem with your motor, you checked to see if the carbs might have picked up a bit of crap from the fuel tank?