OK, I'm stumped...

I didn't see anything about the carbs. Maybe a few clogged secondary jets? when were they last services? or cleaned?

 

I had a yami 90 where upper rpms the fuel line was colapsing causing it to do the same thing. just a thought

 

Would this help? run it up to the speed it bogs down and shut it off, pull the plugs and see if there is fuel on them. then you will know if it is fuel starved.

 

First things first... Before doing anything else I'd want to eliminate your boat's fuel system as the problem.... That's easily done, bring a portable tank with new fuel and by-pass the boat's entire fuel system, hook up directly and run it. If the same problem occurs you're in motor country, if the thing runs clean and green then you've got an issue with your boat...

One thing that can screw up a properly running motor is to have just enough trash in the bottom of a fuel tank to clog the pickup when the demand is at its highest... there's also an anti-syphon valve in most large tanks that can occasionally cause difficulty, and one last little fuel tank gremlin, the clogged fuel vent (if a mud dauber makes its nest in your fuel vent it can restrict or stop it up entirely).

If it does turn out to be your motor, I'd bet on the ignition system breaking down as the rpms get to a given level. That's just a guess on my part, my first move with a suspected motor problem is to find a pro that I trust. He (or she) will have it sorted out in a fraction of the time that I would... Good luck and post up whatever the fix is...

 

if your going to take that 2ed fuel tank you may also want to take a can of Seafoam. After testing if you have the same problem add the sea foam and run that spair tank dry. This wil clean any cloged 2ed jets and blow out any build up in your fuel lines that are in the motor it's self. We would do this at the begianing of evey summer with my off shore boats. One full can of Seafoam in about 3 gals of mixed fuel if you don't still use the oil injector sym.

 

Mabey check your fuel connector from the hose to the motor sometimes they suck air into the system , and when it cuts out does it do it abruptly or does it just break up? If it does cut out abruptly it might be a coil issue

 

i had the same issue on a 2001 130 yammi 2joke, it was a dirty carb and as stupid as this sounds when ur trying to get on a plain or when it starts to bog down try pulling the throttle from neutral to full a few times rapidly and just keep doing that until it gets above 3600 and hold it a full and that helped me, also throw some carb cleaner in there

 

check your fuel/water seperator when it happens could be water in the tank/filter and is not letting enough fuel thru at high speed

or...the "stator/magneto/magnets" (having brain fart)under the fly wheel is defective and the magnets are moving when heated up and throws the timing off----it is hard to determine unless u pull the flywheel immediately after it happens

or fire it up in the dark and see if there is any fireworks on the motor

all of the above happened to me over the years

 

Just fixed an issue on my merc 150 2 stroke (no computer/ carb) that sounds exactly like yours. I know not the same motors but the same technology. Started intermittently missing at high rpms, then progressed to complete bog down past 70% throttle. Mine was a trigger (timing advance under stator)

 

I think Josh may be on to something here. Is the Yamaha using an optical crankshaft sensor?

Frank_S

 

Tom, as you know I have the exact thing going on, fuel delivery is perfect, mechanic thought it might be the power pack. Replaced that, still doing it. I ordered new coils today hoping that might be it, but was told exactly what Josh said, that it might be the trigger or stator. I will let you know as soon as the coils come in and I check it out. And like yours mine came and went away then cme back twice as bad. But mine is on a 25 so not as pricey as yours.