Hope you figure it out. I know you have been hunting this gremlin for awhile.
Have you looked for any air leaks between the carb and air box?
I got these for my pregen ( not sure if they fit yours) because I was sick of the spring clamp, just don't over tighten.
Roadformer 1.75" Worm Gear Hose Clamp - SAE 28 Size, Full Stainless Steel with 1/2" Band Working Range 33mm - 57mm Duct Clamp Pipe Clamp Worm Drive Hose Clamp Fuel Line Clamp (10 Pack, 33mm - 57mm)
https://a.co/d/hPyVWJE