Farmall Cub

I had a similar problem with my first F Cub and I was too lazy to give the gas tank a good cleaning. It finally got me irritated to the point that I removed the tank cleaned it and that completely solved the problem. Good luck with this one.
Berlin

Going with what you've done and a process of elimination it sounds like there is a varnish or whatever coating in the fuel line between the filter and the carb.
That gunk may be flaking away as fresh gas runs through it and that may be re-clogging the carbs jets.
The metal line rarely gets attention when everything else, carb, strainer. tank, gets cleaned.
If its a rubber fuel hose toss it for a new one.
Ever run Seafoam through it?