HI my buddy has a VP 5.7 Gxi in a 24 ft Grady white offshore 24 dual SS prop (not sure of pitch yet as she's in the water ). We noticed it wont hit the rated rpm of 5000-5200 when cruising and when revving the motor in neutral she seems to be bouncing on a rev limiter at 4200-4300 rpm (if the gauge is accurate) he gets a top speed of approx. 43-45 mph @ 4300 rpm , we changed the plugs out with fresh NGK TR55IX plugs (are these correct?) and a distributor cap/rotor The old plugs looked rich and sooty . Do you think the ecu has a lower rev limiter rpm set or could there be another issue?