1991 Yamaha 2-stroke bogging down at WOT

[Roverdude]

Hi,

I have a 1991 Yamaha 25 2-stroke that boggs down at WOT. It runs great at idle and at lower speeds but once I go full throttle(5400rpm) for about 30 seconds it boggs down to a stop. I can hold the throttle in the max position as it does this and it sputters for a few seconds, then all of a sudden back to full speed and then does the same thing over again. Priming the bulb does not help...it stays firm. I started off checking the timing. Then I replaced the fuel diaphragm, the fuel lines, all the gaskets, and spark plugs. The fuel filter was clean so that was not replaced. None of that fixed it so I then cleaned the carbs, replaced the gaskets, and got new floats/needles. The lower carb showed signs of a stuck float so that was replaced. This also did not fix the problem. Next, I replaced the stator because I thought it was a faulty pulser coil. That also did not change the issue. One thing I've noticed is that the motor runs further WOT when the motor is cold versus when fully warmed up.

Does anybody have any thoughts on what to look at/ replace next?

[99yam40]

have you tried monitoring spark and timing when it has a problem?
does no good to do that when running good

[TownsendsFJR1300]

Can you spritz some pre-mix to each carb WHEN it acts up and see if the engine picks up?

If so, that'd point you in a fueling issue..

.

[robert graham]

Make sure your fuel tank is venting properly.....crack open the cap just a bit and try it...

[99yam40]

he said the bulb stays firm, so maybe the motor is over heating, but that should not shut off the motor, just limit RPM to about 2K

[Jason2tpa]

When you cleaned carbs did you also pull and clear jets?

[99yam40]

When you cleaned carbs did you also pull and clear jets?

how would it run properly as long as it does with plugged jets?

[Roverdude]

have you tried monitoring spark and timing when it has a problem?
does no good to do that when running good

Yep, looks like it has good spark on both cylinders. Looks a little stronger on cylinder 1.

[Roverdude]

Can you spritz some pre-mix to each carb WHEN it acts up and see if the engine picks up?

If so, that'd point you in a fueling issue..

.

Motor is on a small gheenoe so that would be quite sketchy. May have to give that a go on a different boat.. It definetely seems like a fuel/ carb issue.

[Roverdude]

Make sure your fuel tank is venting properly.....crack open the cap just a bit and try it...

I also did that. Made no difference at all :/

[Roverdude]

When you cleaned carbs did you also pull and clear jets?

Yes, all jets looked solid. Motor runs super nice, just not wide open!