Sounds like the pop-off pressure on the carb is just too low. The carb inlet valve should be able to stand at least 10psi before being forced open and that is more than the pressure that should be building up in the fuel tank.I believe this pressure build up is causing running issues. I can run it out of a can of gas and it runs fine. But not so with the tank (pressure build up).
I've got to figure out a way to relieve the pressure. I pulled the vent assembly and don't think I saw anything unusual. MAybe have to pull it again. It won't run with this pressure build up.