250 Pro XS stalling at WOT

[1958]

2010 Mercury 250 Pro XS
Serial 1B804914

Motor starts fine and runs fine but at WOT there is a slight sporadic stalling. It runs open, then stalls for a split second, then runs open, then stalls for a split second, over and over. It isn't enough to see a drop in rpm when it stalls but can't get over 5600 rpm. If i back off throttle ever so slightly it runs fine.
Brought it to the mechanic - changed spark plugs, fuel filter, lower unit oil and water impeller. Also drained all gas out of fuel/water separator. No faults showed on computer.
Boat has two gas tanks - it does the same thing on either tank.
Any thoughts?

[Savage]

Click on this >>>Performance problems? Read Here and Upgrade Fuel Supply System FIRST!

All have the steps have the steps have to be performed on the same day in the EXACT order.

[EuropeanAM]

+1

Also, did you get PRINTOUTS of the Run History and Freeze Frame (Fault) History?

That 5600 RPM reading- is it coming from a Smartcraft Instrument or an Analog Tach?

[1958]

Been a while but it's still doing it. Kept kinda hoped it would go ahead a break down completely to show what problem is,
The 5600 rpm is from smartcraft guage.
I did all the steps to Savage's link. Changes hoses, filters, etc.
Every now and then it'll run fine but mostly it won't get to 6000 rpm. Sometimes I feel the hesitation and lunging, sometimes just won't get over 5500 rpm. If I back off on throttle ever so slightly it runs fine and will actually pick up a little speed but never does WOT.
I do all the right things, let motor warm up before taking off, let it idle befoew shutting off, even trim up when idling so exhaust port on back side is not under water, etc....and I get this wierd problem, lol.

[Meadows]

I've been dealing with the same thing all year as well on my 200 ProXS. I still can't find the culprit. I've done basically everything you said that you've done, and still get normal readings on the computer. My normal WOT is usually around 65mph, and right now once it gets to about 60 it starts stalling. If I get mine figured out, I'll be sure to let you know what the deal was.

[EuropeanAM]

Connect a separate, portable fuel tank, with it's OWN fuel line, directly to the Pulse Fuel Pump inlet fitting.

Run the engine on the water and see if the problem persists, or is gone (a simple method to determine if the problem is inside the cowling or in the BOAT'S fuel delivery system).

[1958]

Connect a separate, portable fuel tank, with it's OWN fuel line, directly to the Pulse Fuel Pump inlet fitting.

Run the engine on the water and see if the problem persists, or is gone (a simple method to determine if the problem is inside the cowling or in the BOAT'S fuel delivery system).

I'll do the same meadows
European one mechanic that looked at it ran it with a separate tank but still did it. I've had 3 different mechanic look at it. One thought he had it by changing thermostats but nope

[angle iron]

Have you considered the coils?

[EuropeanAM]

Or a plug wire that's arching (either due to damage, or due to water leaking somewhere inside the cowling).

A CDS recording of this occurring might be helpful... and if you can get that done, I'd also recommend a Cylinder Misfire Test at WOT (limit trim as necessary to keep the boat under control).

[TN-TREY]

I had a problem similar to this on a 225 the inside of my fuel line had separated from ethanol gas and was clogging the fuel flow at high RPMs