newer Yamaha 8hp high thrust bogs, dies, after running for an hour

seamanben

Seaman Apprentice
Joined
Mar 12, 2007
Messages
31
Four-stroke engine is 3 or 4 years old. After trolling with it for an hour or so, the engine starts to bog down, then eventually dies. This happens sooner if I'm running the engine harder. I can get the engine started again, but it only wants to idle, and if I run it again at trolling speed it will bog down and die again. When it cools, it runs fine at trolling speed again.

The engine oil is at the normal level. There is water coming out the **** tube fine. I bought the engine new, and everything under the cover looks fine and not corroded.

The fuel has been winterized, and I'm on a new tank anyways. Gas **** is open when running, no kinks in the fuel line...

Installing a new thermostat is the only thing I can think of, but I'm really not sure... Any ideas of what might be wrong?

Thanks!!
 

seamanben

Seaman Apprentice
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Mar 12, 2007
Messages
31
Re: newer Yamaha 8hp high thrust bogs, dies, after running for an hour

Maybe a sensor of some sort is getting overheated? Or gas is getting pushed through rings to mix with the oil?
 

robert graham

Admiral
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Apr 16, 2009
Messages
6,908
Re: newer Yamaha 8hp high thrust bogs, dies, after running for an hour

Maybe fuel delivery problem, gas tank venting, quick disconnect on motor good and tight, fuel pump O.K., drain carb for water or debris, remove and clean carb?...any of these. Good Luck!
 

seamanben

Seaman Apprentice
Joined
Mar 12, 2007
Messages
31
Re: newer Yamaha 8hp high thrust bogs, dies, after running for an hour

Maybe fuel delivery problem, gas tank venting, quick disconnect on motor good and tight, fuel pump O.K., drain carb for water or debris, remove and clean carb?...any of these. Good Luck!

The thing is, nothing changes between running good for an hour, then bogging down. Gas tank vent is open, there's a good connection with fuel line. It's fine for an hour every time, then it acts up. Which makes me think it doesn't have anything to do with the carb.

I would think it has a lot to do with temperature.. either it's not staying cool enough or a sensor is thinking it's too hot.
 

seamanben

Seaman Apprentice
Joined
Mar 12, 2007
Messages
31
Re: newer Yamaha 8hp high thrust bogs, dies, after running for an hour

It's the 2008 model, by the way.

After running it for half an hour tonight, the **** stream was only slightly warm. I would have figured it would be hot at that point.
 

Cpt.Stubeez

Recruit
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Jul 1, 2011
Messages
1
Re: newer Yamaha 8hp high thrust bogs, dies, after running for an hour

I've got an 2002, 8hp, 4 stroke, Yamaha as well and it is doing the same thing. And i've gone through the same checklist as you...i did pull the plugs and they seemed to have a bit of buildup. I've been running 87 octane which is what was recommended from the manufacturer. But i'm thinking that at low rpm it is putting some buildup on the plugs. So i'm going to start running the no ethanol 91 octane to see how it does. It also seems to stumble on low rpm acceleration so i'm hoping that's the issue. Let me know what you find out with your's.
 

seamanben

Seaman Apprentice
Joined
Mar 12, 2007
Messages
31
Re: newer Yamaha 8hp high thrust bogs, dies, after running for an hour

I've got an 2002, 8hp, 4 stroke, Yamaha as well and it is doing the same thing. And i've gone through the same checklist as you...i did pull the plugs and they seemed to have a bit of buildup. I've been running 87 octane which is what was recommended from the manufacturer. But i'm thinking that at low rpm it is putting some buildup on the plugs. So i'm going to start running the no ethanol 91 octane to see how it does. It also seems to stumble on low rpm acceleration so i'm hoping that's the issue. Let me know what you find out with your's.

I replaced the thermostat on mine. It was really gunked up. After trolling for a few days I could tell the problem was fixed. The boat shop charged me $75 for the part, but I see they are available for $35 online. The replacement took only 10 minutes. hope that helps!
 
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