Have you checked the distributor operation?
I think that it has a centrifugal regulator of the avance, inside the distributor.
If it is ok, check the ancle of the avance or loose the distributor and change the ancle a little while the engine is working, in order to hear the sound.

But if it has smooth sound and it back firing just some times, it may has spontaneus compustion at 40 degrees Befor Top Dead Center. It happens when the pistons has a film of carbon and it is very dangerous for the engine. If this is the reason buy some engine and injection cleaners. put a bottle of that stuff in the gas tank. Do not do that with propan!! only with petrol.
Any question feel free to contact me