I have recently had a strange occurrence whilst out in my 1979 Triumph TR7. I was in fourth gear whilst descending a fairly steep gradient on one of our local roads when I was shocked to hear a loud bang, followed by the sound of a badly blowing exhaust.
I had the exhaust replaced and I’m now concerned in case this problem re- occurs causing me to have to pay out again to replace a perfectly serviceable exhaust system. Andy Turner The symptom you have described is that of the combustion gases igniting in the exhaust system. This normally occurs when the fuel air mixture has not been ignited within the engine cylinder and has been allowed to flow out on the exhaust stroke and into the exhaust pipe.
The cause can be a failure in the ignition system, such as the point or condenser but on the TR7 one of the more common problems was the ignition switch failing. The contacts can make and break causing the loss of ignition for a short while. As you were descending a gradient, it is likely that you wouldn’t have noticed the problem until the contact re-made resulting in the unburnt mixture in the exhaust being ignited by the hot exhaust gases from the re-activated engine. I would first check the ignition switch and test to see if wiggling the ignition key will cause the engine to cut out. If you can find no defects with the ignition system I would recommend you check through the connections to and from the coil and contact points.