I had the same problem with my 67 273 2 brl. Also at hot idle I could watch that clear plastic fuel filter fill and empty repeatedly. So the fuel was peculating in the bowl all the time.
I changed the fuel line to the 73 design that relocates the fuel filter to vertical beside the timing cover and installed the thickest carb base gasket available for that 2 brl. Both problems solved.
I do buy ethanol free locally today but I can run either.
