I would disconnect the fuel line at the carb and see how much fuel is getting to the carburetor, Test the fuel pump pressure. It should be between 2 and 4 psi.
I recently went through a similar situation . It turned out that two new fuel pumps were POSs from China and neither was putting out the requisite amount of fuel. I had the original one rebuilt and it solved the issue.
Could your fuel pump be clogged up with debris from the old gas tank you just removed?
Take the top off and look inside. Maybe there's a bunch of rust or gunk in there from the old tank. The diaphragm could also be torn or stretched from ethanol if it was installed prior to the introduction of ethanol.
