Maybe I should have stated "only encouraging responses".

All kidding aside, I'm now hoping it's "only" piston slap. Yes, KB pistons, due to their silica percentages, expand less, which allows for a tighter wall clearance. Min required is .0015". I don't have the specs what these cylinders measured out at, but they may have been bored for a typical cast piston or even a forged piston which expand much more. If so, this would cause the KB piston to be "looser in the hole" than what is optimum or suggested.

If I put my hand on the throttle linkage to the carburetor when it's idling cold, I can "feel" the knock, so it is substantial.

Has anyone with a piston slapping engine noticed the slap going-away with some RPM even when it's cold?

As far as pulling plug wires one at a time....which I'm glad to try....Should I assume you're suggesting this to eliminate spark knock as the culprit? I don't think it could be spark knock as it goes away with temperature, even at idle, and I'd think it would get worse.

I appreciate the responses given.