Are you using the choke and warm-up lever when attempting to start? Does it start easier after it warms up?
As Bondo said - When you pump the bulb, you're pushing gas through the fuel pump. The fact the motor runs after you prime says the fuel pump is working, Not that it's working well, but good enough to run the motor. Disconnect the fuel line between the pump and carbs, then crank the motor over, Fuel should spurt.
Again as Bondo said - The bogging could be telling you it's time for a carb soak in cleaner and a rebuild. I would rebuild the fuel pump at the same time, it's cheap. Also, check timing and compression.