You need to determine whether this is because there is extra load being placed on the engine
or whether you are not delivering enough fuel/air.
If you try to rev it up in neutral, will it hit maximum governed rpm
? Have you checked to make sure that there is no growth on the prop or shaft? Have you run aground recently or hit anything that could cause bent propeller
blades? Have you done anything else like add significant weight to the boat or any underwater modifications? This should help to figure out whether extra load is being applied to the engine.
The next place to look would be fuel
. You have changed the fuel
filters which is good. Do you know that the fuel itself is good? Did you cut open one of the old filters to see what was in it? How old is it? If your fuel is good, there are a few other things that can limit delivery
. Is your shutdown properly adjusted and traveling the entire way? Have you readjusted your throttle cable? It is possible for a lift
pump to go bad or even an injection pump to cause this problem. If you hook up a fuel pressure gauge between the lift
pump and injection pump, that should help you. Your engine also has a "smoke limiter" which monitors boost pressure in the intake and limits fuel output if it is too low. It could even be that there is something wrong with your governor.
The next place to look is air. You will probably be able to see more black smoke than usual if you are having an air issue. Likely problems would be a dirty air filter, a boost leak or a worn out turbo. Boost leaks
can be frustrating to find and usually are not realized until a boost gauge is connected. To check your turbo bearings, grab the compressor
and see whether the shaft has play. Reduced air in itself won't reduce power that much but it will cause your smoke limiter to reduce fuel output.
This is a laundry
list of things to look at. Start with the obvious and see whether you are loading the engine more heavily. Then, if you don't see black smoke and if the engine is running well (no air in fuel), start looking for fuel related problems.