As long as there is no backfire if you back up the engine from compression before starting it Paul, it seems reasonably certain that the backfire is being caused by the failure to crank. When we fix the failure to crank, the backfire should be gone.
There are ways to investigate, for instance, how far the engine rotates backward when it fires just as it comes to rest, about 20 degrees before Top Dead Center. To do this without modifying the engine for the experiment, however, would require high speed movie cameras or a similar technique. If it were done, it would probably tell us with certainty which of two things is happening. The first possibility is that, when the engine is rotated backwards by the force of combustion pressure to a point midway through compression, the worn-out decompressor opens the exhaust valve by a tiny amount, causing a backfire. The second possibility is that the backward rotation extends a bit further, to the first 20 per cent of the compression stroke, when the intake valve is still slightly open, causing a pop in the air intake and air cleaner. If I were there to see and hear you crank your engine, I could probably tell whether it rotates backward when it pops, and if so, how far. I could probably also tell whether the pop is produced in the muffler or the air cleaner. However I'm not there, and it doesn't matter anyway, as long as it is cured by repairing your engine, which has to be done anyway. Producing those pops is damaging your engine, and you should stop doing it.
