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Old 04-19-2019, 03:46 PM   #1
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Yanmar GM's

I may be seatrialing a boat with a couple of 3cyl diesel straight inboards installed.
These are inter. rated at 3600 rpm/ continuous rated near 3500. The owner claims both do 3000 at WOT, but no excess smoke.
Can someone check my plan?

1) make sure props and hull are clean
2) insure full travel of throttle cables are achieved. I think I should be able to see the travel stop on the engines.
3) With warm engines, do a high idle test, likely with a handheld tach.
4) If high idle does not achieve 3600, then adjust it.
5) Now what to do if high idle IS achieved without adjustment? Compression test?
6) If compression is good, reduce prop pitch?

anything missing?
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Old 04-19-2019, 04:27 PM   #2
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Reduce prop pitch.

I had an Albin 25 before the Willard and most of the A25’s that were repowered had the 3GM30 engine. Mine was powered by a Yanmar but a bit bigger at 34hp. The 3GM (at 27hp) was known as the go to engine for repower. Everybody thought they were wonderful.
I cruised my GM at 2750rpm for some time and the only problem I had was an annoying seawater leak at the front end of the heat exchanger.
But don’t worry about the high engine speeds. These engines are designed for it. They should be fully capable of reaching their rated rpm. And IMO a tad more is better than a tad less.
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Old 04-19-2019, 05:33 PM   #3
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The Yanmar GM engines are often over propped, but also a very common problem may keep it from reaching 3,600 rpm. The raw water exhaust injection elbow plugs with soot and scale and that results in a back pressure that limits wot rpms. Remove the raw water hose and poke a screw driver blade in the water inlet and see if you can feel scale/soot build up.


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Old 04-19-2019, 05:42 PM   #4
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David,
Yes I remember there was considerable trouble with the water injection elbow. In the 6 yrs I was with that engine seems to me I replaced the elbow more than once. Cleaned it a couple of times too.
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Old 04-19-2019, 09:27 PM   #5
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Dave,

I just checked my copy of the Yanmar Diesel Inboard Shop Manual where I found the following regarding high idle.

Model Full throttle rpm (no-load)
3GM 3750
3GMF 3750
3GM30 3825
3GMF30 3825

In addition to the points you propose in your plan, I would:

Insure the engines are completely cold (overnight) before starting. The GM series engines are notoriously difficult to start if the compression is not up to spec due to engine wear etc. If they start easily when cold, that's a great initial indicator of condition.

Ask when the valves were last adjusted. That is supposed to happen every 500 hours and can affect WOT performance/power.

Ask when the fuel filters were last replaced.

Insure air filters are clean.

That's all I can think of right now.
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