I have a 2015 Mercury 4-stroke outboard on my boat.  The last couple times, when I'm up on plane at a high RPM, the outboard is losing power.  It throttles down for a second or two, and then comes back up.  Its not a big loss of power, but its noticeable.  I have only ever put ethanol-free premium gasoline in it, and I've gotten it professionally winterized every year.  Any idea what the problem could be?  I did some online searches and I think it might simply just be a bad spark plug.

Fuel supply problem, could be a filter or prime bulb leaking air.

Tom

There’s a real good chance that it’s a dirty fuel filter. I had the same thing happen with me this spring when I got my Suzuki out, in the water separator in mine, there’s a small filter that had some microscopic particles in it. Enough fuel flowed through to start perfect and get on plane, but after about 3000rpm, it’d start to bog down. As soon as I changed the filter, it ran perfect and has all season. It’s amazing how such a little amount of particulate can choke it off.  

Thanks will check those items

That motor should burn E10 with no problem what so ever.  My Yamaha is 15 years old and does and there has been no fuel line or primer bulb deterioration.

Do you run Quick Kleen and Quick Care in your motor?

Don't know if that Q is for me, but I only use Yamaha Ring Free with every fill up since I bought the boat.( .5oz per 5 gallons) per the manual.

Sounds like it's not getting enough fuel at high rpm.  As said above, definitely check for leaks in all fuel lines/fittings, replace the fuel filter on the engine, and replace the water separator filter if you have one.  Also check the mechanical fuel pump(s) and make sure the check valves are working and the diaphragm isn't leaking.  I do know that on some Mercurys, the pumps use these odd ring-shaped pieces as check valves, which can sometimes break and need to be replaced.  Opening up and inspecting the pump(s) is worth doing for that reason.

 

Not sure if your engine is carbureted or fuel injected..

 

If carbureted, it's possible the main jets are slightly clogged.  You can try running it at a high idle with the intake silencer removed, and hold a rag over each carb intake for a few seconds.  Sometimes, this creates enough vacuum to free up a partially clogged jet.  It's also possible that the needle valves and/or floats are damaged or gummed up. To inspect/repair that would require you to take apart the carburetors.

 

If fuel injected, it's possible that the injectors are clogged.  First try a fuel injection treatment through the fuel tank (Lucas, SeaFoam and many others make these treatments specifically for fuel injectors).  If that doesn't help, then I would remove the injectors and send them in to be professionally cleaned/repaired.  It shouldn't be too expensive, and worth doing just to get new o-rings and baskets.  If that doesn't help, then it's possible that the vapor separator chamber isn't staying full (due to a bad mechanical fuel pump or needle valve), or its internal filter is clogged.  In that case, you would need to take apart the vapor separator.  Another possibility would be a bad pressure regulator.