I have a 2007 Formula 382 Fastech with twin Mercury 700 SCi's and NXT drives. The motors used to turn 5,200 RPM (+/-). Now BOTH motors will only turn 4,000 RPM (+/-). The only changes made between when the motors ran 5,200 and 4,000 is removal of the anti-siphon valve from the fuel system, replacement of the fuel water separators and replacement of all spark plugs in both motors.

I have run through 3 tanks of fuel since the motors would only turn 4,000 RPM, so I do not believe it is the quality of the fuel. The boat is stored on a trailer so it is not build up on the bottom of the boat.

My mechanic has also plugged into the motors and they show 100% throttle and are not in guardian mode. No codes have been set.

Anyone have any insight as to what may be causing this issue?

Thank you!

What gap did you set the plugs to?

I would look into what you changed and work backwards. I would start with checking fuel pressure (anti siphon valve and fuel filters).