It's hard to troubleshoot when you get no feedback from the original poster. I pretty much rule out things like crossed plug wires or other mistakes because he didn't say it started after he did such and such. He also said he rode it home after it did the backfire thing and it was fine until the next day or so. To me, it sounds like the carbs are leaking and dumping raw gas into the cylinders. Cranking shoves that gas into the exhaust and the slow burning gas in the cylinders ignites the gas in the exhaust and there you have it. Just like redwing suggested. Does the 1200 have a vacuum operated petcock like the 1500? I'm gonna say his fuel is not shutting off.