I'll take this at face value and say you had an backfire and it came through the airbox. If this was the case then I would look at the carbs and see if any seals were blown out or if the diaphragms were ruptured. I would also check the floats and see if there is any crud. Turn the gas off and take one of the bowls off. Turn the gas back on and see if you have flow. Then gently push up on the float to see when the gas stops. Do the same for the other side. But avoid a mess and put the other bowl back on.
. The top of the floats should be about even with the bowl mating surface of the carb. Ruff estimate.
I've had this happen too with miss adjusted valves, jumped timing and bad ignition.. But since it was running fine and you were fiddling with carbs I would start there. If you do the above and all seems well, I would re-do the carb adjustments, and check to see if the cable runs are correct if you had the tank off.