Mercury 250 Pro XS 4 stroke surging

I have just turned 100 hours on my 2020, 250 Pro XS 4 stroke. This past Saturday as I was cruising across the water my engine started surging. I could slow down and it would still surge on me. This has never happened before.
My first guess is I have water in the fuel? I have always used Sea Foam in my tank
My second guess is one of my spark plugs has fouled? I'm sure spark plugs last more than 100 hours?
My third guess ...well I do not have a third guess.
I called my local BPS to have them do a 100 hour service and check on the engine, they guy there said that it would cost between $600 to $900 for the 100 hour service and the engine would most likely fall under warranty unless it was something like a spark plug of fuel filter.

Any of you all had this problem and what was the fix?

Try spark plugs first, their know to foul early with a lot of idling and low rpm operation. Also not covered under warranty so BPS will bill you an outrageous amount.

Yea I know…..I can change the spark plugs out myself.
I changed my oil and change out the lower gear oil.
I can get the NGK plugs for 1/2 of what BPS is asking for them then around $90 to put them in.

if the surge is at an idle, i'd suspect the fuel first.
run a fuel sample.

also.. be sure to change the fuel filter.

600$ for a 100hr service is absolutely crazy over priced.

and no.. it's not unusual to see plugs fouled at 100hrs , especially if you do alot of idling. ... run it harder.
also.. be sure to gap the plugs when you install them. on a 250 v8 ... .035 is the gap

Dang, I just glad to see I am not the only one that thinks 600 bucks for a 100 hour service is way to pricey lol.

Remember the $600 is the low end.
I have bought a new fuel filter and 8 spark plugs,,that was over $200
I will put them in maybe tomorrow evening and give it a run Saturday. I have my year end classic championship November 12th. On Lay Lake, I will be doing some longer runs to find those long creeks.

something else to look into. on a 4 stroke w/ a fuel supply module.. you should NOT have a primer bulb in the fuel line. and most boats have a fuel demand valve on the fuel tank outlet or in line.. those can cause a problem.

Hey Steve,
Would that also cause it not starting very well? My engine fires right up and roars like it should. No spitting or stuttering at all. It only happens when I’m on plane.
I took my fuel filter off and replaced it. I blew through it and it was not even close to being stopped up.
I took my spark plugs out and replaced them also. I did not notice anything different from all eight of them. A nice white color burn and the tip and electrode was not worn one bit.
Hopefully tomorrow I can take some time to get on the lake and see if everything I have done so far has fixed the problem.
If not I will have to take it to a shop and get it fixed, hopefully under warranty. I still have 3 years left on my warranty.Then it will take two to three weeks to get it back if they even have the part in stock. If not it could take a lot longer so they said.

Took the boat out today and tested it. It seems I have taken around 90% of the surging out. I still felt it at top end around 6k rpm.
Come off the throttle just a bit and everything is fine.
I can live with that until I can get it into a shop and have all the electrical sensors tested. This engine is ate up with sensors. I think it has sensors that tell sensors what to do.
I have just about empty the tank of gas tank is below 1/4 tank. I will put about 30 or 40 gallons in it plus another can of sea foam to see if it was water in the tank.

You’re not using regular seafoam are you?

It is the white and red bottle
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