Bf200 surging, spark plug issue ?

My first post, been following a lot of content for years. So thanks for any input!

I have a Worldcat 266 with twin Honda bf200 (2008)
on one of my motors I have a really weird 3800-4000 rpm surging like it’s starving for fuel. Falls on its face flat if you try to add throttle. But rpms slowly decrease if you just “leave it”
wierd thing is fuel pressure is at 45psi and all fuel lines/filters check out.(new) but the problem only happens after running for 45mins - 1 hour. So that rules a fuel problem/filter out in my mind. (That’s about 10 gallons gone through the system)

checking everything yesterday I pulled the plugs for curiosity and half of them the tips were almost gone. These are NGK IFR6k11 (should be compatible with IFR6F.)I replaced these plugs beginning of the year maintenance.

im going to throw the old NGK IFR6F in this weekend and try it out- but if sparkplugs are bad it would cause surging right away right?

my question why are plugs tips almost gone?and chalky white(maybe 50hours on them? Tells me it’s running to lean?)I checked the ecm by jumping the plug but no codes-—
And surging after an hour of running only? I don’t get it.
thanks for the responses.
unfortunately shops are 6 weeks out at a minimum to look at a boat. And I have a feeling It’s something easy.

Try another O2 sensor, they can cause this very issue

Tested fuel pressure on a different gauge, it reads 40/41 while idling with vacuum hose off and plugged(wasn’t doing this before but was getting the same reading)
is it worth while leaving/testing fuel pressure when the surging happens?
i also switched 02 sensors from good engine to this one.

Definitely, fuel pressure must be monitored whilst fault is active

Let us know what happened when you put the old (and correct) plugs back in. Also, what were the results when you swapped ECU's? Do you run the boat until the engine faulted using the replaced ECU?

The tips being "burned off" the "K" NGK plugs would indicate to me either a lean burn, or the plugs burning too hot (I'm not sure of the differences between the NGK "K" plugs and the correct "F" plugs.) And if they were really "burned off" I would suspect bad performance at all rpm's.

Another thing that could cause that is water in the fuel. What did the plug electrodes look like - they should have a light brown coating if burning properly. Do you have a separate external fuel/water separators on each engine or one common one from a single tank feeding both engines? In either case when were they last drained/changed? Have you dumped the fuel bowl on the onboard F/W separator? Have you drained the VST?

If possible, connect a clear plastic hose to the primer bulb and run it directly to the inlet to the low pressure fuel filter. Run the engine until it falters and look for air bubbles.

You said that you did not get any fault codes. However, I would check for fault codes again. If the MAP, IAT, or IAC sensors were going bad, that should show up as a fault code. Here is the correct procedure: View attachment Procedure for Getting Fault Codes.pdf

It would be useful if you could post a pic of the spark plugs

Chawk- thanks for the reply I see you help ALOt of people with great information over the years, it’s nice to see your help/reply to my post.

i have never had water in the fuel/ I have a Parker pre filter prior to Honda filters. They were just changed as well.
i did not switch ECUs . The bulb is firm when the surging happens.
my next outing i have switched 02 sensors and will run a auxiliary tank with a clear line to see about air bubbles. I will also by pass the garmin Fuel meters as well.
i will post pictures up of the plugs on my next post.
Again thanks for the help all-

Update-
Hard to work on the boat with 3 kids under 3-
anycase I had 2 new aluminum fuel tanks made as the old ones had pin hole all over the fittings on top. Definitely probably letting air in the system on the one motor that was surging.

my question is how much if ANY air is allowed in the system and what is normal.? I am pulling my hair out tracking down where it’s coming from. I have a auxiliary fuel pump that I hookup to different areas of the system and can cycle fuel without the motor on.
-coming straight off the tank no air
-coming off my Racor filters I’ll get a steady stream of fuel for a couple seconds than a big burst of bubbles. (Rebuilt/resealed fittings) it’s better but still noticeable amount of air.
plumbing from the tank to the Honda filters is less air but still small steady burst of bubbles . Tried different hose with with out clamps.
I get bubbles on both engine even the one that never had problems. So am I chasing a ghost???
thanks all

Try a portable tank on the troublesome engine to rule out fuel supply. There will always be a little fuel separation in the line with vacuum, especially if you have anti-syphon valves in your fuel pick up.

Agree with Ian. Those surges of bubbles can also be caused by a failing seal on top of the fuel bowl in the external fuel/water separators. Enough draw pressure builds up that it pulls air past the weak seal into the fuel bowl. Take a look. and when you re-install or replace them, coat the big O ring with some Vaseline.

Dropping in- I’m pretty sure I Fixed the problem

first And foremost my aluminum tank was replaced, all the original welded on barbed fittings for hoses were corroded and some had even pin holes in them.
New tanks were built and all new fuel lines.
When I pumped the gas out of the original tank(100gals) or so maybe 4-5 gallons of solid water was also extracted. My problem engines fuel system was full of water at all points. But never set off the Honda separator/filter.
my Racor/pre filter was full of water as well.
I had just change these the beginning of the season so I was not thinking about checking or replacing this the last thing on my mind was a water in fuel issue . I had only put on about 24 hours after filters were changed

Took boat out today, fresh fuel/ fresh filters and ran it hard
never a hick up. I have full confidence in it again.

Thanks for all the help and info.

Side note—- water was concentrated in the racor but the fuel in VSt/Honda separator was not. I had to introduce water and shake it in a container(cheap ethanol test) and it pulled the water out.

Easily 50-60% water by the time it hit the motor but it was in suspension with the fuel- I can’t get pure gas, I get shell down my street and it’s E10. So some of it was ethanol??

And for the surges of air- it was the plastic tubing, I had all sizes tried all of them, extremely sensitive to how there attached.
only worked if I used a hose clamp. . They don’t conform tightly like the rubber/honda hoses do on the fuel filter barbs.

One of of the best 10$ I bought was. 12v diaphragm pump off amazon. Easy way to pull gas through the lines without turning the motor on.

E10 should be avoided in marine use if at all possible due to its hygroscopic character.