Mercury 90 Loss of power above 4200 RPM

[danport]

I have a new to me 2009 Mercury 90 hp, 2 stroke, ELPTO 1090412FY serial # 1B780051

It starts well, idles pretty well, a little rough.
It accelerates well and powers up quickly to about 4200 RPM, which is maybe 3/4 throttle.
If I push the throttle past 4200 rpm the engine loses a small amount of power and the rpm drops off a little to about 4000. Boat speed stays the same or drops about 1 mph.

Top speed on my 20 foot pontoon is 17 mph at 4200 rpm. It is not a tritoon, no lifting strakes, 25 inch toons. I was expecting a top speed of about 22 to 25 mph.

Compression test shows 120, 118, 120

On the fuel side of things I have:
emptied the fuel tank and replaced with 91 octane alcohol free gas with 1 oz seafoam per gallon.
checked the fuel tank vent.
checked the fuel pickup in the tank, the screen was clean.
replaced the gray fuel line to the engine with the black automotive fuel line.
replaced the primer bulb, and it primes normally with the bulb firm after a few squeezes.
replaced the fuel lines between the carburetors.
cleaned all 3 carburetors, but they all looked clean with no debris and no blocked jets.
replaced the fuel filter twice
rebuilt the fuel pump, but did not find any problems, the membrane was intact.
temporarily put in a clear fuel line section just before the engine to check for air leaks, I did not see any bubbles in the line.

After cleaning the carbs, I followed the Mercury service manual for the link and sync procedure, and I checked the timing. If I remember correctly, at cranking speed 5 degrees BTDC for idle and 20 degrees BTDC for maximum timing. Anyway set to the factory specs.

I checked that the carb throttle valves are opening fully at WOT.

On the spark side I have:
checked the spark gap, on all 3 cylinders the spark will jump a 1/2 inch gap idling on muffs
replaced the spark plugs with new NGK BUZHW-2 as recommended on the engine sticker

Replaced the impeller, and I get a good water stream from the tell-tale.

I believe this engine does NOT have a rev limiter.

I tried raising the engine on the transom (propeller higher in the water) with no improvement.
I have experimented with changing the trim while underway.

My prop is 14x13P. I have NOT tried switching props, but the Mercury prop selector calculator suggests 13.75x15 or 13.25x16 so I do not think I am over propped.

At this point I am not sure what is next. I have ordered a DVA meter, direct voltage adapter, to try to do some testing on the CDM modules (coils), the stator and trigger. This motor does NOT have a switchbox.

At this point I could use some help and suggestions, please.

 

[Texasmark]

Nobody hit on this so I will toss in a bit. Fuel starvation would be my first guess so I would feel the bulb which should be soft, and then squeeze it several times to see if the RPMs increase which would indicate lacking adequate fuel supply to engine to run to higher RPMs.

Next, what are the conditions of your floats? if you have any algae buildup it can severely restrict your top end. With that extra load of algae drag and your current prop, the engine may be loaded down and 4200 is all that it can put out unless you change something....cleaning the floats or decrease the pitch of your prop.

 

[danport]

Nobody hit on this so I will toss in a bit. Fuel starvation would be my first guess so I would feel the bulb which should be soft, and then squeeze it several times to see if the RPMs increase which would indicate lacking adequate fuel supply to engine to run to higher RPMs.

Next, what are the conditions of your floats? if you have any algae buildup it can severely restrict your top end. With that extra load of algae drag and your current prop, the engine may be loaded down and 4200 is all that it can put out unless you change something....cleaning the floats or decrease the pitch of your prop.

Thank you for the reply. I have tried having a buddy pump the primer bulb while running and it made no difference. The primer bulb is new and pumps gas well and firms up normally when squeezing it. I though fuel starvation too, but I have redone basically the whole fuel system.
The pods have no algae on them. They do have some sort of a mineral like flat crusty spots on them that feel sort of like sandpaper, I didn't think that would affect it this much but maybe.

 

[danport]

Here are pictures of the surface of the pontoons. I am starting to wonder if this rough surface could make a lot of difference. It looks like caked on mud, but it does not wipe off.

 

[Texasmark]

Thank you for the reply. I have tried having a buddy pump the primer bulb while running and it made no difference. The primer bulb is new and pumps gas well and firms up normally when squeezing it. I though fuel starvation too, but I have redone basically the whole fuel system.
The pods have no algae on them. They do have some sort of a mineral like flat crusty spots on them that feel sort of like sandpaper, I didn't think that would affect it this much but maybe.

Well your stiff bulb proves that fuel isn't the issue. Those floats look horrible. Get them back to new and shiny and you will be amazed at the difference in top end!