i'd bypass the existing fuel system and hook up a line to the carburetor from an alternative fuel supply.
I usually use a squeeze bottle with gasoline in it and four or five feet of fuel line. I suspend it higher than the carb to ensure gravity flow. Fire up the engine and see how long it will run using secondary fuel supply. I it keeps running, the problem may be the vacuum powered fuel pump if it has one.
If it has gravity flow fuel supply, check the entire system for debris. Pull the line off the carb and see how much fuel comes out of the line. If not a good, solid flow, there could be blockage in the tank?
If spark and fuel are both good, I'd look at valve adjustment, and potential air intake, exhaust restriction.