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Old 28-04-2019, 16:05   #1
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Yanmar 2qm20; Why Low Revs?

I'm looking for reasons my 2qm20 seems to run at a lower rev range than specified.
The manual shows max 20 hp output at 2600rpm.
My engine cruises comfortably all day at 1400rpm and achieves hull speed at between 1500-1600rpm. At full throttle the engine approaches 2000rpm, makes black smoke and sounds like its going to blow up pretty soon. Free revving gets it just over 2000rpm.
I've considered 2 possibilities; over propped and/or inaccurate rev counter. The prop and engine configuration is the original fit from 1979, not that that means it was correct even at that time; the tach is also original and has given the same readings for 10 years.
To my ear the engine sounds pretty comfortable operating in its present range but I have always been bothered by the theoretical discrepancy.
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Old 28-04-2019, 16:10   #2
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Re: Yanmar 2qm20; Why Low Revs?

Black smoke and under revving is almost always a case of over propping. I would also confirm the tach is correct.
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Old 28-04-2019, 18:58   #3
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Re: Yanmar 2qm20; Why Low Revs?

If your engine only reaches just over 2000 rpm in neutral you have another problem. Assuming tach is correct it could be any number of things I'd check fuel flow, throttle & governor linkages, compression, air flow,injection timing & tappets.
You MUST be able to reach max revs in neutral before you start messing with prop size.
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Old 28-04-2019, 20:02   #4
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Re: Yanmar 2qm20; Why Low Revs?

Whoops forgot to add blocked exhaust /manifold, a common problem.
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Old 28-04-2019, 20:43   #5
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Re: Yanmar 2qm20; Why Low Revs?

If you are getting hull speed at 1500 rpms then your tach is way off. Get a strobe and check it.
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Old 29-04-2019, 09:02   #6
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Re: Yanmar 2qm20; Why Low Revs?

Over propped it's not bad at all, less revolution means some diesel savings
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Old 29-04-2019, 09:15   #7
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Re: Yanmar 2qm20; Why Low Revs?

Saving a LITTLE on diesel cost is a drop in the bucket compared to the cost of the damage you will cause to the engine when running it over propped !! DO NOT run it in the RPM range where you are getting black smoke !! It must rev to its max. rated RPM in neutral then address the potential over-propping issue.
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Old 29-04-2019, 09:30   #8
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Re: Yanmar 2qm20; Why Low Revs?

Quote:
Originally Posted by Manrico View Post
Over propped it's not bad at all, less revolution means some diesel savings
You're kidding, right? If over-propping was good, every boat mfg would sell over-propped boats and hype it up in their marketing and say what a good thing they are doing to help their customers. Guess what...they don't. They follow engine specs.

Engine mfg's spend millions designing and testing their engines. After all this they come out with specs for cruise speeds and max WOT RPM's and they tell you the dangers of both over and under propping.

Go over to boatdiesel.com and start a thread about how good over-propping is and see what kind of response you get.
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Old 29-04-2019, 09:40   #9
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Re: Yanmar 2qm20; Why Low Revs?

I've wondered about the working engine speeds myself sometimes with the Yanmar engines that I've owned.

I've had 2GM's and now an older 3QM30H. These engines seem to work just fine operating in the 19-2100 RPM range. I've tried the diesel sweet spot "70% max RPM" rule-of-thumb on a few occasions (around 25-2700 in these cases) and always end up backing off until the engine "sounds and feels" more comfortable. Other than keeping up with the usual maintenance (filters, belts, zincs, oil, etc.) I've never encountered an issue with the engines at all, not even once. In fact, on both types that I have owned, I could pretty much achieve near hull speed and maximum fuel efficiency at the same time.

There could very well be underlying issues with your drive train and prop but, now at least, you have a bit of a yardstick from someone else to compare things to, regarding lower than expected RPMs.

While I do think you have a Tach issue (1400 does sound a little low), if your engine is happy, you're not building carbon or belching smoke and your fuel consumption is efficient, carry on. Most manufacturer's specs are wordsmithed to present the best possible but not generally realistic scenario; in this case, maximum horsepower output.
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Old 29-04-2019, 10:11   #10
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Re: Yanmar 2qm20; Why Low Revs?

See-Breeze1, nothing wrong with backing off and running at a lower RPM. But when the engine mfg specs call for a max WOT RPM of, say 3000 and all you can get is 2000, you have a serious problem that needs to be fixed.
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Old 29-04-2019, 11:06   #11
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Re: Yanmar 2qm20; Why Low Revs?

If it has worked fine since 79, I wouldn't make a mountain out of it. Black smoke means intake blockage, perhaps exhaust restrictions. Overpropping isn't bad for your engine unless you are at full throttle then exhaust temps can get too high, not good. But under propping is worse as exhaust temps aren't high enough to prevent a gooey carbon buildup on your injectors and valves, also not good. Plus as mentioned overpropped saves a lot of fuel. I added 4" pitch to mine dropping the rpms 500.
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Old 29-04-2019, 11:26   #12
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Re: Yanmar 2qm20; Why Low Revs?

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Originally Posted by SteveSadler View Post
. Plus as mentioned overpropped saves a lot of fuel. I added 4" pitch to mine dropping the rpms 500.
I really have a problem believing you are saving fuel by overpropping. Think about it. Let's just use WOT figures, but the same logic applies to the full RPM range. You have an engine that burns 2 gallons per hour at WOT of 3000 RPM's with the correct prop. Now, let's overprop the boat so it now only gets 2500 RPM's. Guess what? You are still burning 2 gallons per hour. Why? Because you are running WOT, meaning the engine is burning 2 gph whether it's turning 3000 or 1000 RPM's or anything in between. The throttle is WIDE OPEN.

Now run at 70% throttle and let's say you are burning 1.2 gph. Whether you are running 2100 RPM's with the correct prop or 1750, overpropped, you are still burning 1.2 gph.
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Old 29-04-2019, 12:26   #13
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Re: Yanmar 2qm20; Why Low Revs?

Quote:
Originally Posted by Manrico View Post
Over propped it's not bad at all, less revolution means some diesel savings
What rubbish! The fuel you use with an over propped set up, at those lower RPM will in all probability exceed that which you'll use with the engine running at its designed speed. You'll also end up with a lot of carbon build up in the exhaust ports and manifold.
Diesel engines rely on a plentiful supply of air being pumped through them to attain good, complete, combustion.
Can't tell you how many engines I've serviced with such problems.
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Old 29-04-2019, 12:57   #14
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Re: Yanmar 2qm20; Why Low Revs?

Quote:
Originally Posted by SeaBreeze-1 View Post
I've wondered about the working engine speeds myself sometimes with the Yanmar engines that I've owned.

I've had 2GM's and now an older 3QM30H. These engines seem to work just fine operating in the 19-2100 RPM range. I've tried the diesel sweet spot "70% max RPM" rule-of-thumb on a few occasions (around 25-2700 in these cases) and always end up backing off until the engine "sounds and feels" more comfortable. Other than keeping up with the usual maintenance (filters, belts, zincs, oil, etc.) I've never encountered an issue with the engines at all, not even once. In fact, on both types that I have owned, I could pretty much achieve near hull speed and maximum fuel efficiency at the same time.

There could very well be underlying issues with your drive train and prop but, now at least, you have a bit of a yardstick from someone else to compare things to, regarding lower than expected RPMs.

While I do think you have a Tach issue (1400 does sound a little low), if your engine is happy, you're not building carbon or belching smoke and your fuel consumption is efficient, carry on. Most manufacturer's specs are wordsmithed to present the best possible but not generally realistic scenario; in this case, maximum horsepower output.

I do appreciate others experience of these older engines, please chime in if you have some;
The thing about the tach - at slow idle the engine sounds about right - 500 - 700rpm, so I dont think the tach is necessarily inaccurate. - I suspect the max rev of 2600 in the manual may have been a bit high. Also suspect most people are used to the later model engines that rev to the mid 3000's so my rev speeds sound pretty low. I must say also that the prop has been pushing this boat without complaint for 35 years so I'm not looking for any basic structural problems - there aren't any.
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Old 29-04-2019, 13:47   #15
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Re: Yanmar 2qm20; Why Low Revs?

I had a Yanmar 2gm20 in my 31ft Miura and spent many months trying to work out why the motor would not rev (no rev counter) above what seemed to be "too slow". Opening the throttle beyond a certain point simply resulted in black smoke. Eventually a friend produced a hand-help rev counter and we tested the motor with the boat tied to the jetty. Fortunately we tried both forward and reverse gears - BINGO, the motor would only reach approx 2000 rpm in forward gear before black smoke appeared BUT in reverse gear it reached 3500. "When in doubt, read the manual": it turns out that there is a major difference in forward and reverse (reduction) gears, approx 2.1:1 in forward but 3.1:1 in reverse. Finally we realized the truth, the prop was too course for forward gear, but fine for reverse. Not much use though!
Andrew MacKenzie, Cape Town.
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