Is it backfiring or is it just air coming back thru the carb. I had a problem with a rebuilt engine, it was weak running and it did not backfire, it blew air back out the carb. No one here could help. Followed their advice and wound up spending hundreds of dollars on parts. From rebuilding dist, carb, adjusting valves, replacing carb. I just could not get anyone to agree that the cam was either bad, or too retarded. Well the last is what it was the mach shop had the cam too retarded. Advanced the cam and walla has mucho nuts now. Need more info from you. The post before mine has excellent websites for ID purposes.