Ditto!!!!!!!!Had the same problem with my 83 aspencade, took the carbs apart and found the slow jet plugged. after a good cleaning and it stopped for a while but then happened again, crabs back apart and jet pluggged also float just a bit off. Repalced slow jets with new reset floats and presto no backfire. If i forget to turn the fuel on and start it, as soon as the fule level drops enough in the bowls it will backfire but that is all. Probably a bit of carbon in that cylinder and the lean condition heats it enough to preignite during intake I am guessing.