remove the fuel cap, and see if you still have issues starting it. if not, you have a tank vent clogged. if still problems, your primer pump could be leaking fuel, and letting air into the system. also, all the small lines on each injector could be letting air in, or one of the fuel lines from the tank to the pump... also, your fuel filter 0-rings could be bad.
a simple way to find leaking fuel lines, is to put a shop vac in blow mode, and stuff it into the fuel filler neck, then look for fuel leaking out of hoses.