Is your fuel pump good or is 100 years old? Also the fact that it does it with both carbs makes ya think it isn't geting enough fuel to the carb for some reason. Restricted fuel line faulty pump etc. I assume the float level is correct.
If you are allowed maybe put a fuel pressure gauge with an isolator , so you can watch the fuel pressure while you are running it.That would tell you if it was the carb or the fuel system.