If you remove the fuel line from the carb check the flow of fuel from the line. It should flow with the speed of a full line, If you are getting little or no flow then there is a restriction either in the line or the outlet of the tank. Could also be a restricted fuel cap causing loss of flow, but that should allow it to start back up after a few seconds of removing the fuel cap..
Could also be a restriction between the fuel inlet on the carb, and the float valve.