Guys I was kidding. There is not nearly enough information in post #1 to make anything but a wild guess. I thought my wild-guess comment would be funny, but I guess not. So I'm sorry to be a dick.
So the number one go-to with a backfire is;
A) if it backfired through the muffler it's probably ignition.
B) if it popped through the carb, it probably went lean.
And it always makes sense to check the ignition first cuz it is waaaaay easier to fix. So check the plugs and check the timing.The plugs are like little windows into the chambers. Every one tells a story, and it is really nice when they all tell the same one. While they are out it makes sense to do a compression test right away to. And don't stop at 5 or 7 or whatever revolutions. Stop when two consecutive pulses read the same or very nearly the same. And then, next to the pressure, record the number of pulses it took to peak.
After the engine is proven to be mechanically sound, and that the ignition is doing it's job, and that the plugs are capable of lighting the fires; then you can attack the carbs.
Yeah, you might just have a plugged air bleed.. Or water in the gas. Or any of a dozen carb problems. But guess what; you can chase a carb problem around for days, when the problem was a faulty valve spring, or slipped timing, either of which is a quick find with the previous tests. And when you have two carbs, it can be a lot of hunting.
