I have a Donzi 28 ZX with two 350 Mag MPI's that I have just replaced with Michigan Motorz GM base marine engines. The engines I bought the boat with had not been winterized correctly and both had cracked blocks. The new engines run very well with one exception. The last 2 inches of throttle travel on the sticks are causing the engines to bog slightly slowing the boat down. I have fuel pressure guages on the dash and they are reading 40lbs (measured at the end of the fuel rail) when the bog takes place so I don't think I have a fuel issue. I am using aeromotive adjustable fuel pressure regulators instead of the stock ones. The boat does have colder thermostats in it (120 degrees) but I wouldn't think that would cause the bogging I'm experiencing. According to the Donzi guys this boat should run in the high 60's or low 70's with this set up. I'm only getting 61mph from it right now. Is there a sensor that could cause this, bad TPS maybe?

that is a cold azz thermostat. m 350mpi's have 160* stats. verify the thermostats you have.

What RPM does the bog happen?

When the throttle is at idle are the throttle body blades closed and conversely do you get WOT with the throttle advanced all the way? In other words is the rigging set correctly?

The boat has 120 degree thermostats from Procharger. The boat is geting WOT with the throttle body.

Are the regulators vac operated?

So you have Prochargers on these motors?

If you have ProChargers that changes the entire equation. That has to be answered first.

No the engines are not Procharged currently and the regulators are hooked up to manifold vacuum.

What rpm does the bog happen at?

It could be the TPS sensor. You can check the voltage off the sensor and it should run from basically 0 to 5 Volts as the throttle is swept.

If this boat had ProChargers at one time were the ECU's reprogrammed for the boosted application? You may still be running a SC-Tune with 2-bar MAP sensors. IF these are the only factors a re-flash will fix you up.