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Old 04-09-2013, 03:52   #31
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Re: Yanmar Horsepower

Originally Posted by Boracay View Post
Ok, so if we're guessing why the engine lost revs I'll put my money (but only a few shekels) on an over pitched prop.

Most of the time it would make no difference but if full power were applied to an over pitched prop then the engine may overheat and some power would be lost.


My betting is an intermittent clogging and then clearing of the exhaust elbow. An easy part to remove and check (assuming the engine is above the waterline!!) Just ensure that the thruhull is shut!

If not, another option is debris in the tank that occassionally blocks the supply pipe.

A long shot would be a problem with a sticking injector

Check the exhaust elbow, and then dose your tank with a triple dose of Fuel Set.

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Old 04-09-2013, 04:18   #32
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We have a Yanmar 4JH3E. The rated max RPM is 3,800 but like Our Doug we don't usually run over 3,000. I have seen a "loss of RPM" before but it was the throttle cable system slipping. It seems that this engine has a tendency to "push back" on the throttle when revved above 3,000. So in our case it was a mechanical thing.

If the prop is over pitched it should cause some dark smoke. Nothing like that was reported. I doubt the exhaust elbow can intermittently clog. It is possible the pickup tube was getting temporarily clogged but usually the symptom is the engine revs up and then dies.

Next time you have a chance do a long straight run. Push the revs up to 3,500 for a couple minutes and see if they hold. If revs fall off carefully look at the throttle linkage to be sure the cable is not slipping slightly.

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Old 04-09-2013, 07:14   #33
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Re: Yanmar Horsepower

It is still most likely fuel starvation.

If you are running against a tide race with a following sea and the boat is pitching about, sediment in the bottom of the fuel tank may be stirred up and partially clog the fuel filters this, in combination with the sludgy fuel starves the engine of fuel. When the vessel motions decrease the sediment and sludge settles out and things re fuel supply return to normal and the engine will be able to achieve full revs.
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Old 04-09-2013, 14:52   #34
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Re: Yanmar Horsepower

Thank you, yes I feel fuel starvation is most likely.
Perhaps time to clean out the tanks!chaeck filters, etc.
Very good to have all the feedback .

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