I have a 50 HP 4JH engine which is suffering about a 30-40% loss of
power. I cleaned the fuel, changed the Racor and the engine

To me it sounds like not enough fuel pump pressure. The engine burns more fuel
under load than when freewheeling.

Check your fuel pump, filters and the connections in the hoses for
restrictions. Some trash might be in the fuel hoses. If the Injection pump is
bad usually the engine will miss when freewheeling at WOT.