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Old 18 June 2013, 11:08   #1
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Problem with Yamaha impeller

Hi can anyone help,my Yamaha 40 hp pumps water OK when cold,but after a while as it starts to worm up the flow gets less and less. Is this as the impeller worms up and gets softer,and if so will a new one stop this.
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Old 18 June 2013, 12:19   #2
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No, it's something else. Does the water get really hot?
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Old 18 June 2013, 12:34   #3
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Not that hot just worm i think.
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Old 18 June 2013, 13:03   #4
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Impeller is well away from the powerhead; only heating will be from friction, which is pretty unlikely as it's submerged in water.

Have you checked the thermostat? And made sure the telltale hose is clear?

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Old 18 June 2013, 13:30   #5
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Perhaps the thermostat is opening too soon allowing water to circulate further in waterways and never getting that hot.
Or needing to be flushed of all salt/crud etc. Poke thick fishing line down the tell-tell hole to make sure that's clear of crud.
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Old 19 June 2013, 08:34   #6
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Thanks to everyone that has got back to me,got going with it last night and gave it all a good clean out, thermostat and behind the exhaust cover, what a mess it was in dirt sand and grit all back together now, and just gave it a run it was fine but as it started to heat up, and I picked the revs up the water flow started to slow and get very hot,is it the thermostat that is at fault .
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Old 19 June 2013, 15:32   #7
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According to my F115 shop manual, the thermostat should be closed at 28-52 degrees C Above 60, it should open at least 4.3mm. While specs may be different for your motor, the operation should be fairly close.

One other thing to think about is that if you had that much crap behind the thermostat, what's actually in the water journals? Maybe a good flush and descaling would be in order?

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Old 19 June 2013, 15:34   #8
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If that horse it's old place a new thmo, probably with issues already, other thing. If engine was poorly rinsed after use in salt water, you may have built slat, crust formations inside water paths, depending on how much it's clogged, salted inside, will eventually experiment higher than normal temp exiting through prop & pee hole.

Crankcase it's not being refrigerated by impeller as it should, as when having clean water paths. If changing a new thermo doesn't fix the issue, will be in need to pull cylinder head off and perform a exhaustive water paths clean out, take advantage to decarbon head, head pistons too. Place a new head gasket, torque to specs, will have a revitalized running wild horse again.

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Old 19 June 2013, 15:49   #9
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I noticed one thing on my Yam 75 impellor casing, that a little 'tang' got sheared off from the stainless impellor case, which meant it would rotate with the impellor inside the plastic housing, and I 'thought' reduce flow .. although I never followed up my theory, I just replaced it ... do you have any pics ?
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Old 19 June 2013, 16:04   #10
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Before you go tearing into the powerhead...

Check that the little metal tab that holds the impeller in place is working right. It could be that the impeller is slipping round the shaft at higher revs and output. And at idle the torque on the impeller is limited such that it pumps ok.

I'd replace the impeller before doing anything else. Its cheap and you may discover the source of the issue inside the water pump.
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