Awfully hard to say without being there, but as a general rule.... if it pops but won't run there is not enough fuel getting to the cylinders. To check this, you could remove the spark plugs, put a few drops (not a squirt) of gas in each plug hole. Reinstall plugs and try to fire right away. If it tries to run a little better than before, you have your answer. Good luck! If it backfires out one of the carbs, it is probably 180° out on the timing, Like Dan says.