Hi Terry,
Looks like Sean has set me up on this! Actually, I think it is quite a simple issue to explain. Where the exhaust leaves the manifold, there is an elbow with two tubes which engage with another elbow on the hull side. They are intentionally free because there is slight movement in the engine mountings. When new, the swaged ends of tubes are snug in the elbows, but after a long time the elbows wear inside and the steel tube tends to flake a bit on the swages making them loose. On the overrun, is where you get the backfire and this is a sure symptom of an air leak in the exhaust system. What can be done? Well you could find the modern equivalent of asbestos tape and bind around the tubes, or renew them. I have had success in the past by building up the swage by brazing and filing back until the just slide in the elbow with no slack.
regards, Richard