Re: Unpredictable transition from low speed to high speed

[Don]

Your video shows the engine running great at WOT, but doesn’t show it running poorly at low speed. You swapped out the carb and it runs the same.
You mention that it idles poorly, then sputters before finally getting to high speeds.
So, my guess is that it is running on one cylinder while idling, maybe only getting spark to one cylinder until the throttle is advanced to WOT. You may want to try those inline neon testers while the engine is idling, or do a cranking spark test.
Did you try new plugs?