2018 DF200 suddenly running rough and under power, ie cavitation

[MColeman]

I have a 2018 DF200 in a 2019 tri-toon. The motor has been running great all sumner. I had to let the boat sit for about a month recently. Went to start the motor and first noticed what seemed to be a “slow start” line the battery might be low. The motor did start though.
After about 5 min of running at low speed the motor began to cavitation and shake. I ran the motor a bit longer. I usually can reach around 40mph. At full throttle I can only get to 27mph now.
After reading a few threads I decided to visually inspect the fuel filter located toward the back of the motor. I noticed that while there is fuel in the filter, the level of fuel in the filter is only about 2/3 full. In other words, the filter is not completely full when running idle and I can see the fuel “sloshing around.”
My first though is a clogged filter or bad fuel. I did add a fuel stabilizer.
One additional note, when I was running full throttle, again only achieving a consistent max of 27mph, at I e very brief poi t, just a second or so, the speed picked up to to what I would say is normal, around 40mph. That said, I also experienced the opposite where the speed dropped even below the 27 to say 20 or so. Again these were very brief moments.
The batteries are fully charged according to the multi-meter and a charger I have on hand.
Any thoughts?

[Murray]

Try adding Seafoam or a similar product, you may have some moisture in the fuel. It can’t hurt and is a simple/cheap fix if it works.

[artdf175]

fuel filter? racor type or the in-line filter ?
either way there should be no air ..
it indicates to me a high probability of a loose fuel hose.
the other thing it might be is a plugged fuel tank vent - which in turn creates a vacuum and the engine starts starving for fuel
Art

[Pettechfl]

Mine did the same thing, I had a 2017 Df 200. Cylinder wall failed at 100hrs. Warranty wont cover it either.

[Moonlighter]

#1 task is to inspect the breather outlet for the boats fuel tank. Mud wasps like to build nests in there and block the vent….which creates a vacuum in the tank and starves the engine of fuel

# 2 task is to inspect the main water separating fuel filter that should be fitted between the tank and the engine, after the primer bulb. Eg the Racor style filter. Remove the filter element and tip the contents into a container and check it for water and contaminants. Install a new filter element regardless.

If your vessel does not have such a filter, then shame on your boat dealer/supplier. Because now you probably have contaminated fuel all through your fuel system and it will need to be cleaned out and all engine mounted filters changed (some of which are quite costly) and the VST cleaned out and reset. Expect a hefty bill.