I assume you were pulling gas out of the can under the hood using the stock fuel pump. If that's true, it sounds like the pump is O.K. If not, how was the gas getting from the underhood can to the carb?
Often, I've found that the line is not necessarily plugged but that it has rusted through somewhere and the pump is sucking air instead of gas.
If the pump is pulling from the underhood can, but not from the can when placed in the back it almost has to be the line--plugged or rusted through. Try disconnecting the line from both ends and shoot some air from a compressor through the line--you might hear a hiss from a rusted through hole or verify that it is plugged--it may even clear the line!
just my 2 cents...
