It sounds like the pilot jetting in those carbs may be lean. That's the idle mixture. So when you accelerate, the main jet takes over, which may be fine but would be rich compared the pilot if the pilot is lean. The transition back to idle momentarily causes fuel starvation and the high oxygen condition that results in the exhaust for that few milliseconds ignites unburnt gases and causes the "pop". True, it is a delicate balance. Given that there are 2 carbs, they also need to be balanced and as such, it could be only one causing the issue. It may be as simple a balancing them so that one is not more or less open or closed than the other.