The Rochester B models have been vilified greatly around here and in many cases they're 100% deserving of the complaints. But I doubt this backfiring is due to the Rochester. I believe Jon got it right. Somehow fire is occurring at the same time an intake valve is open and that's what you need to stop. I'm not certain why running it overly rich would make it run any better. Many Rochesters run overly rich naturally due to their basic design.
You can check manifold vacuum (not ported vacuum) at idle for a quick view of things. Adjust the distributor so as to (more or less) maximize it. Once you've done that see if the vacuum needle is steady or fluctuating and go from there. If vacuum drops in an intermittent fashion at idle, one (or more) valves could be sticking, hanging open or maybe burned. If vacuum is steady while idling but lower than normal, your timing may be retarded. If it is higher than you think it should be at idle, your timing may be overly advanced. See what you see and let us know. Good luck. Sorry you ran over your air cleaner.
