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Hello, my name is***** will assist you.
The problem sounds like it is going to be a plugged filter or the fuel line running from the boat to the engine. When these lines begin to fail internally, they will block the fuel flow to the engine over time. Start by replacing all fuel lines from the boat fuel tank going to the engine, and replace the engine low pressure fuel filter.
Your going to need to test the fuel pressure on the engine while it is running. Make sure that it is not dropping below 34 PSI. Also run a compression test on the engine. It should be over 180 PSI on all cylinders with the throttle opened.
Engine compression cannot be tested on a running engine. This is done in the shop with all spark plugs removed, the throttle opened slightly and a compression test gauge screwed into one of the cylinders at a time. Crank the engine over until the gauge reaches its max reading for that cylinder. this should be done for all 4 cylinders. Below is what a normal test kit looks like
Ok, when the fuel pressure, that is causing the engine RPM to drop as well. Feel the Vapor Sep Tank on the side of the engine to see if it is getting very warm where the fuel pump is located. If so, the pump is failing.
Run the engine on another fuel tank to see if the issue goes away.
Do you recall what the compression numbers on the engine were?
Is the engine losing spark or the alarm sounding when this happens?
Does pumping the primer bulb over and over while the engine is starting to slow help or hurt the issue?
Has this engine been sitting for some time?
Yes, sticking injectors can flood the engine. you would also have fuel in the oil and fouled spark plugs.
You wrote that the fuel pressure falls off as the RPM fall. You will need to test the fuel pressure regulator, and see if there is any debris inside the FSM.
Sorry, on your engine it is the VST. this is where the electric fuel pump is located. Is that the one that you have replaced?
Have you run the engine on a separate fuel source, including the fuel line from the tank to the engine?
Alarm ever activate?
How is the water flow from the engine?
There is a filter in the high pressure line, was that also replaced?
What year is your engine, or the full model number. I will get the diagram for you.
have you replaced it already?
Ok, How is the voltage on the engine?
If the voltage is falling when the engine is running, that will drop the power and can slow down the pump.
I have to run to a dr appt. back in about an hour.
do you have a computer hooked to the engine?
did the timing change as the engine warmed up or if throttle is applied?