Need a Mopar guru: 273 part-throttle ping driving me crazy!

67Dart273 said
From the page you mentioned: Is this what you are referring to?

"The manifold was made from cast metal. The molten metal was poured into a mold through a little hole and when the manifold was finished, the little hole was plugged up with a little rubber plug. Well, after a few years this little plug would dry up, shrink, and fall out, leaving a hole in the manifold."

Yes.

67Dart273 said
I've never heard of such a thing as a factory rubber plug. YOU MIGHT HAVE an early (72) intake which DID NOT use an EGR valve, BUT DID have EGR. The first year or two that Mopar used EGR, Ma just drilled into the bottom of the intake, into the heat crossover, which of course is exhaust, and "jetted" it for a continuous leak of exhaust into the intake. On those early manifolds, YOU MAY or may not be able to see them with a flashlight down through the carb throats. You may have to look down through the bores with the carb removed, see below

Once again, these early EGR manifolds HAVE NO external EGR valve. No external clues that I know of

OK. I am a little confused as you are saying no EGR valve but did have EGR... Let me see if I understand your point: Are you saying my 273 has an intake manifold with a "hole" in it that is allowing/leaking exhaust into the intake? If so, how would I plug it (looks like a thread plug? in your pic above) and would it even be possible with the carb removed?

So the recirculated exhaust could be causing a lean condition?

67Dart273 said
You mentioned very high octane fuel. ARE YOU CERTAIN that what you hear is spark knock and not some other noise?

I am not ABSOLUTELY certain, but let me answer you another way (not trying to be a wise-guy here): what else would cause a knocking sound at part throttle to improve when I used a mix of 100 octane with 91/92 octane? Feel silly for asking in light of your question, but... could there be an issue with valve adjustment?