Backfiring is hardly ever a timing issue, it is for the most part always a lean mixture issue. At least on these two bangers it turns out. Try enriching the idle mixture screw a good bit and watch that backfire go completely away as you just begin to chuff the faintest wisps of black smoke.
I guarantee it!!
What happens is not a too early an ignition at all, but one that doesn't even happen until it's way way too late, when the exhaust valve is opening. The other cylinder is lighting the missed one's contents as it's pushed into the muffler. It didn't catch fire in it's own cylinder because it was too lean, but it will burn when exposed to fire from the working cylinder. The fire will even run backwards thru the opening exhaust valve and set off that mostly confined charge and make a real ker-bang out of it on occasion. It's not really backfiring at all, because if it did that you would get smoke and gasoline spray out of your air cleaner stack and the engine would stall because the massive air flow thru the carb backwards would be a situation that would be unrecoverable except at highest RPMs. Gas would be blown out the carb vents as well and would drip all over.
It does sound like a backfire sure enough, but it really isn't one at all. It's a complete dead miss getting burnt in the muffler for the most part. But the right mixture will catch fire just like it's supposed to and more importantly WHEN it's supposed to.