Cleaned Out The Vst And Hooked A Fuel Pressure Gauge Off The Fuel Pump And Turned On Key And The Gauge Went Over 60 Psi, Not Sure If I Checked It Right But Does This Have Fuel Regulator And Is It Under The Plenum?

The fuel pressure reg is mounted on the end of the fuel rail. It should be around 40 so you have an issue. It sounds like you may be stuck somewhat closed and when you need it to close all the way (high load) it can't, so you lose fuel pressure.

On my 502MPI there was a schrader valve to measure fuel pressure on the front of the motor (on the front of the "inlet plenum") by the fuel pressure regulator for the rail. Should be about 36-40 lbs. Did you run the boat since cleaning the screen on the VST?

Also, did you start the motor on the hose and get 60 psig constant or just turned on the key like you said. Motor needs to be running to check fuel pressure.

today took the boat to the lake and my buddies boat used parts off his like fuel pump and relays and ect. the boat starts cutting out at 3800rpm like a spark plug breaking down but hold it WOT and trim up it will rev up to 4500-4600 with that breaking down let off under 3800rpm runs fine, my dad thinks i should try changing spark plugs again incase there is a defected plug, any other ideas?

Had a buddy with a 502MPI that had a chaffed wire for the fuel pump and had similar symptoms. We put a (big) hot wire to the + side of the fuel pump from the battery and the problem went away.

Another possibility, check the magnet on the distributor shaft to make sure it is not cracked. Have to take the distributor out and remove the gear to pull the shaft out but a cracked magnet will make the motor run bad. Mine would not idle well with the cracked magnet but could be a possibility for you.

Did you use the correct coil when you changed the coil?

Did you find the screen in the VST and clean it specifically? What is the fuel pressure at 3800 rpm and up? Did you use your buddies VST?

I would think that plugs would make a miss all the way through the rpm range but changing plugs again will not hurt.

Can you check for codes on the ECM?

Clogged injectors? There are places that can flow injectors, I'm sure someone can chime in.

502 EFI w/VST
 

VST's have been prone to problems with debris in the fuel supply since day one. I've posted this several times but it's worth another post. Install a RACOR filter after your standard Merc water separator fuel filter and before the fuel inlet to the VST and your VST debris problems will be gone forever.