[Excaliber]

Change of heat range is addressing the "symptoms" not the cause, as MOST plugs are firing correctly. Only a few cylinders are affected, not all. I would have to assume the plug wires are same for all cylinders and in good shape. A weak spark to these cylinders would result in your description, hmmmm,,,not likely that, but don't over rule weak spark to affected cylinders (wires, short in cap etc.).

Incomplete combustion,,,,hmmmm....valves set to tight, loss of compression could cause that. Ruling out other factors like, piston rings etc. must be the top end (carb, intake, valves).

Easy enough to move "rich" plugs to "clean" holes. Clean plugs should "foul" just like old plugs did, and "old" plugs should "clean up" in the good cylinder. This would verify that the "bad" plugs are NOT the problem (only a symptom).

Possibly running rich ONLY to affected cylinders, doubt it. Sure sounds like a valve lash problem. Good luck, I'm drained!

Ernie