[Cal Metal]

It is hard to say what may have upset the performance. Those seem like innocuous changes. Your mains are really too lean, though. I am not saying this is contributing to your back fire problem, but 145s should be about the smallest jet that should be used, assuming a 37mm choke and nothing smaller.

I would start at the beginning and make sure the timing is advanced the way it should be, and that valves are correctly adjusted.