[DanEC]

Kevin - I second Lippy's advice. Is it backfiring through the exhaust or the carb - sounds like through the exhaust? If you run it again see if the carb boosters are dripping any fuel into the running engine. I was working on one of my carbs lately and managed to get the secondary float to hang up and it was dripping fuel out the boosters and causing a rough idle and odd exhaust note. As noted by others a leaking intake manifold is all too common with these motors - should be apparent from the plugs. Sometimes an external intake leak can be traced with propane torch gas - but if the leak is internal to the engine it won't work. Good luck tracking the problem.