I had a .40SF that did the same thing in my Super Sportster and after tearing my hair out and I have two things that worked for me. Well, actually only one of them worked, but I did them both at the same time and I didn't want to figure out which one was the magic cure. First I put a piece of fuel tubing around the high speed needle to catch any air leaks that there might be. Second I put a one way check valve on the muffler pressure line so the pressure would remain in the tank and push fuel to the carb no matter what the throttle setting was. I did both of these and it cured my problem. I did the same things you did, changed the tank, moved the tank, tried different plugs, different settings, different everything and this was the only thing that worked. Fourmost makes check valves good for glow fuel. My first thought was you have a hole in the clunk line but since you have changed the tank, that shouldn't be the problem.

Good luck and I hope you find it!