Small Engines (Lawn Mowers, etc.): B&S 5HP fires and dies., air vent, fuel cap

Question
My 5hp B&S (murray is the name brand) I don't have the serial number, mower was returned to me and it will only run if you constantly prime it,will intermittently run for a short period of time without constantly priming, I've tried running it without air filter, without muffler, i changed the spark plug and drained all the gas and added fresh fuel.i can barely keep it running if im constantly pressing the primer "bulb" i was told it ran out of fuel and would not restart(inlaws borrowed it) lol

Answer
By pushing the prime constantly to keep the engine running you have become a fuel pump.  

Check the air vent in the fuel cap.  Check the fuel line, is fuel flowing freely?

For some reason fuel is not going up the nozzle and into the Venturi of the carb.  

If it isn't any of above remove carb disassemble clean and inspect the parts.  If they look worn or damaged replace them.  Be sure to install new gaskets and air air filter. This should fix the problem