1986 sportcraft

boat runs for 2 to3 min. and then acts like its out of gas . i have changed the distrib its electronic, new coil cheked out fuel pump took out the check valve in tank. still same problem i dont know what to do next?????? also changed the fuel filter and it looked good no dirt took out the washable screen in carb no dirt the boat is a sportcraft 305 230hp stait inboard