86 searay with 5.7 mercruiser 4 barrell rochester problem at full throttle the engine pulsates like it is not getting enough fuel runs fine at 3/4 throttle problem started after installing new fuel tank the tank was custum built exactly like the original with all the same size vents and pickup tube. new marine fuel pump installed 6 months ago no bypass from vent tube.pulled the fuel filter after run and it is only half full I think insalling a check valve will help that which it is supposed to have anyway.Any suggestions would be greatly appreciated!!!!!