The issue is that the engine will not go above 3500 rpm’s with out sputtering in and out. I’m not by no means a boat or car mechanic so you will have to dumb everything down for me to comprehend, this is my first boat and have no clue about anything, I have changed out the plugs, wires ,fuel filter and had the oil changed also added a gallon of seafoam to the fuel when half full to try to help clean out the fuel injectors, any suggestion on what would be causing this