yamaha 40hp losing power

[Billwill4811]

My 2004 Yamaha 40hp outboard loses power when under load. After about 15 minutes of operation the engine will lose power and revs only when a high load is put on the engine. For example, when towing a tube and coming out of a circle, or when motoring through small waves or swell. To stop this problem i simply come back to idle, turn the engine off, wait a minute, restart the engine and away i go without any power loss problem. After about another 5 minutes the problem comes back and so on. Without a load on the engine, ie, planing across a smooth lake, the engine never loses power or revs. The marine mechanic has replaced the CDI unit, spark plugs, the coils and fitted a second hand stator assembly to it and still the problem remains. I have used a different fuel tank in case the under floor tank wasn't breathing. I don;t think it is starving of fuel as i can pump the fuel primer bulb while the engine is playing up and it stays hard. It doesn't seem to be isolated to one cylinder either.Does anyone have a suggestion as to what could be the cause? 3 Cylinder, two stroke oil injected.
Regards.
Brett

[99yam40]

low oil level or overheat will cause RPM reduction to around 2K.
Do you have gauges that would indicate one of those if it came in?
Do you have a buzzer that works indicating a alarm situation has come in?
If you do , does anything indicate one of those comes in?
I would be best to post a complete motor model # when giving info

[robert graham]

The "loss of power under load" could sound like a spun prop....do the revs increase during this loss of power under load?....

[Billwill4811]

I have a LCD panel with oil level, engine temp and RPM on it and these indicators are not coming on. The RPM's reduce as well when it loses power. It is a significant loss in power. I will be planing at 24 knots at max throttle towing a tube, ( can't remember what rpm is), and when the problem starts i come down to about 12 knots off the plane at max throttle. When the problem occurs i have removed the cowling and pulled off individual plug leads to try an isolate a particular cylinder but each cylinder has a similar power drop. Each plug lead seems to have a decent spark coming out of it when i pull them off.
Any other thoughts very much appreciated.
Regards
Brett

[Billwill4811]

One thing I was reading in another forum was about the 'fuel enrichment valve' failing causing power loss issues. There is a little red lever near the solonoid that apparently shuts the fuel enrichment off. I will take the boat for a run and see if this could be the problem. If failing it could be allowing too much fuel into the middle cylinder, potentially flooding it? Fingers crossed. I will post back with the outcome next week.
Brett

[robert graham]

One thing I was reading in another forum was about the 'fuel enrichment valve' failing causing power loss issues. There is a little red lever near the solonoid that apparently shuts the fuel enrichment off. I will take the boat for a run and see if this could be the problem. If failing it could be allowing too much fuel into the middle cylinder, potentially flooding it? Fingers crossed. I will post back with the outcome next week.
Brett

It could be that....easy thing to check. If too much fuel in that cylinder maybe spark plug would indicate a rich/carbon condition?....

[99yam40]

If the needle does not seat/close off in the enrichment system then the extra fuel will go to all cylinders,
But if a hole develops in diaphragm the fuel could be sucked into the pulse port maybe. Fuel pump could do this also
I do not see this happens and then going away though

I would monitor the timing while the problem shows up to see it that is changing.

Seems like something in the carb bowl could move around and cover the main jet restricting fuel to a cylinder under those conditions. Once you shut it down it could float off the jet.
Something to look for is all else is good.

Pay attention the what the RPMs , to see what it was and what it drops down to

[robert graham]

If the needle does not seat/close off in the enrichment system then the extra fuel will go to all cylinders,
But if a hole develops in diaphragm the fuel could be sucked into the pulse port maybe. Fuel pump could do this also
I do not see this happens and then going away though

I would monitor the timing while the problem shows up to see it that is changing.

Seems like something in the carb bowl could move around and cover the main jet restricting fuel to a cylinder under those conditions. Once you shut it down it could float off the jet.
Something to look for is all else is good.

Pay attention the what the RPMs , to see what it was and what it drops down to

Yep, maybe drain each carb float chamber onto a clean white rag to inspect for trash/debris there....good idea...

[Billwill4811]

Well, I closed off the fuel enrichment and the problem is still there. I could feel the engine surging a bit before the problem started. I was pulling off spark plug leads while playing up and the bottom cylinder had quite a significant greater power loss than either of the other two. Middle and top cylinders had about the same effect. Seems like the problem is related to the middle and/or top cylinders.