More thoughts:
1) Wonder if you could use the idle speed screw and kinda “sneak up” on the rpm where the trouble starts, but not so far that you kill it, or that it’s repeatedly backfiring.
Start pulling one plug wire at a time and see if RPM changes. A “no change” after pulling a wire is reason to suspect a miss at that cylinder. Bad wire. Valve issue. Something. I’ve located misses that way before.
2) Another check I’ve heard about but never personally tried: Run the engine somewhere pretty dark. Use a spray bottle to mist water around the plug wires and see if you can see arcing across wires.
Again, in your case, I think that might be best done at an RPM where it’s running noticeably rough. If it still idles fine it could be a problem that only presents once the distributor turns a bit.
Last edited by JW51; 12/06/2023 8:00 PM.
