Fresh data:

By "factory settings," I mean the float levels and accelerator pump height. No other changes to the internals.

I reset the initial timing to 8 degrees from 12 BTDC and also (don't laugh) moved the distributor vacuum line from the manifold to timed side. That's been wrong for over 20 years, but the engine ran fine, anyway. Go figure.

Manifold vacuum is 17-18.

Closing or opening the idle jets has little effect - manifold vacuum rises slightly, but engine speed barely changes, even with the jets completely closed! I sprayed carb and choke cleaner around the carb base for leak detection and found none; the idle speed didn't change.

As I had reported above, the engine ran great after installing the new NGK plugs, but quickly (less than 30 miles) returned to running poorly; it sounds and feels as if one or more cylinders are not firing. After gapping down to .035 and road testing as suggested, plugs 1, 4 and 6 are clean and light brown. Plugs 2, 3, 7 and 8 are sooty, 5 is actually wet!

Before I take the carburetor off yet again, does this appear to be a carburetor or ignition problem? The distributor is tucked into the firewall and very difficult to access, but not impossible. I can't remove it without pulling the engine, but I can probably replace the cap and rotor, and have replaced the points and condenser less than 10,000 miles ago, but that was in the mid-2000's.

The carb has been on the engine since 1988 and I'm not averse to replacing it with a shiny new 1405. I'd opt for the calibration kit if it weren't for the fact that this is a very recent problem.