That's a lot of options! Let's see if we can help narrow it down a bit. Here's what I am thinking...
Unless you have a filter vacuum gauge, it might be time to do a filter change, especially if you have been in rough water
recently since that can stir up tank sediment. However a clogged filter is most likely to first start showing problems under high fuel use condition such as motoring near hull speed
. A filter vacuum gauge will eliminate the guess work on filter condition.
engines generally don't accelerate as well as gasoline engines, especially older diesels. The turbo greatly improves diesel acceleration. That's why most diesel cars are turbocharged. If your turbo is not working properly acceleration will if poor, even under low load situations (like in neutral). If the engine wont accelerate (rev) well in neutral it is not bottom growth or prop causing the issue. Also if revving in neutral is a problem it's likely not a filter issue because it's not a high fuel use situation (unless the filters are almost totally blocked). More info on the acceleration problem would be helpful. Revving the engine to near max rated rpm
in neutral to test this is not bad for your engine.
You can test the idea of lack of air by temporarily removing the filter if it has one and opening the engine compartment while the engine is running to see if that improves the problem. Many marine engines only have a screen
and not a filter. Usually there is not much dust in a marine environment
It could be a clogged exhaust elbow
. Often, but not always, you will have reduced cooling water
flow with a clogged elbow. How does the cooling
water flow look? Normal? Low flow would be a sign it's the elbow. Also, if you have not done it in the last year or two, it is time for the elbow to be removed and inspected. They often will develop cracks when they get old which can leak exhaust in to to cabin