Are you sure the vent on the tank is open?????
Sounds like a fuel supply issue, to check the fuel pump, disconnect a hose from the pump to the carbs and crank the engine, fuel should spurt out with each revolution.
If the tank vent is closed it will not let air replace the fuel being used and after a while stop the fuel from leaving the tank
if you can pump up the primer bulb until it gets hard the bulb is probably OK, be sure to hold that bulb upright when you pump it up so it gets all gas in the line and no air
Once the engine is running the primer just becomes part of the fuel hose and is no longer firm. If the engine starts and runs for some time before it quits it would suspect the tank vent. If it starts and runs for just a few minutes and then quits I would suspect the fuel pump