Engine runs fine all the way up to 4K rpm but after that it gradually slows down and just about the time you realize you are going slower, the power comes back. What’s up with that? Any ideas on what to check first?

No change in fuel pressure during this and it just feels like it’s starving for fuel. Total rpm lose is about 300 while running. Rough water, smooth water, doesn’t matter. Half to full tank of gas also. The only thing I have noticed is my fuel pressure according to my gage has dropped by 2 or 2.5 psi over the last three weekends. Could air temps and humidity changing cause that?

By the way I have a Gen VI 502 MPI. AZ Speed & Marine Tuner Kit: MSD ignition, K & N air filter, adjustable fuel regulator, reprogrammed ECU. That was installed 40 engine hours ago with a set of Dana Marine Flo-Torque manifolds and problem just now showing up.

Thanks for you help and I need it!

Change out the fuel water separator, make sure your vent line is clear too.