72 Duster, Ethanol gas, intake backfire

Nice car!

At idle, your effective compression ratio could be 5/1 or even less.
At WOT, it will be whatever the design might be, say somewhere between 7.5 and 8.5.
When you tip in the throttle, the effective compression ratio will vary between those two extremes, depeding on the throttle-opening.
>If for some reason, an intake valve to not quite closed; it might idle just fine. But as the effective C/R rises, the burning mixture, and rising pressure, in the chamber could back up into the intake.
But with log manifolds;
>Then again,an overly rich mixture, still burning in a log manifold can force combustion in an adjacent cylinder,that is on it's overlap cycle, and idling very slowly which allows plenty of time for this to occur.
>And of course the simplest reason of all; the 1-2 whammy of a lean missfire and a leak at the flange of a log-manifold. This goes back to the preceding paragraph, but does not require the slow idle speed. With a missfire, a great deal or all of the just inducted mixture never catches fire or escapes burning in the chamber, for whatever reason. Now it's hot, and entering into a hotter still environment.... that may have fresh air in it. If a still burning mixture, or when,lol, from another cylinder comes along and lights it up, that adjacent cylinder is about to go into overlap, which will directly connect that burning expanding , higher than atmospheric pressure, ball of fire, to the lower than atmospheric pressure intake manifold......... which itself is full of a combustible mixture. Thankfully the overlap period is very small, but the fire in the intake has to go somewhere.............