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Old 03-07-2012, 01:05   #1
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Yanmar 2qm20 - RPM Question

my yanmar 2qm20 runs about hull speed (about 7knots for 29 ft waterline) at 1600rpm according to the tach. but flat out i can get 1900 with quite a bit of black smoke. lowest idle i can get on the tach is about 550 before the eng. conks. I'm wondering if the tach is accurate given the highest revs i get without a load is 2300. Also, i have a new set of injector nozzles, black smoke makes me think i should fit them, any ideas/help?
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Old 03-07-2012, 02:53   #2
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my yanmar 2qm20 runs about hull speed (about 7knots for 29 ft waterline) at 1600rpm according to the tach. but flat out i can get 1900 with quite a bit of black smoke. lowest idle i can get on the tach is about 550 before the eng. conks. I'm wondering if the tach is accurate given the highest revs i get without a load is 2300. Also, i have a new set of injector nozzles, black smoke makes me think i should fit them, any ideas/help?
Classically overpitched prop...

You can use a strobe to check the tach but I doubt that is the problem or if in fact therre is any problem at all.

An overpitched prop will be unable to reach max rpm, it will drive the boat faster for a given rpm and accelerate more slowly. When the prop and bottom are fouled it will be more susceptible to lugging and producing black smoke as the engine can't get up on the torque curve. It also makes slow speed handling difficult as thenboat might go too fast even at idle or just off idle.

You may be able to have your prop repitched. This will give better acceleration, be less sensitive to bottom and prop fouling and may improve slow speed boat handling.

Your question about the accuracy of the tach is interesting as the engine should reve quite a bit higher with no load. Have you physically checked that the throttle pushes to the stop on the injector pump? Also I presume the intake is clear and the exhaust is in good shape.
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Old 03-07-2012, 03:10   #3
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Thumbs up Re: yanmar 2qm20 - rpm question

It should rev to 3800 if out of gear, yanmar rev counts tend to over read, it will probably indicate about 4000 rpm, they are excellent motors but "like" to be at higher rev's
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Old 03-07-2012, 05:50   #4
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Re: yanmar 2qm20 - rpm question

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Originally Posted by boris View Post
It should rev to 3800 if out of gear, yanmar rev counts tend to over read, it will probably indicate about 4000 rpm, they are excellent motors but "like" to be at higher rev's
Hmm... I thought a 2qm20 would max out around 3,000 with 2,600 for continuous operation.
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Old 03-07-2012, 05:54   #5
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A 2qm will not turn 3800.
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Old 03-07-2012, 06:25   #6
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Re: yanmar 2qm20 - rpm question

FWIW: Yanmar “2QM” Series Operator’s Manual

http://www.g-o.dk/Files/Billeder/PDF...QM20-3QM30.pdf
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Old 03-07-2012, 06:31   #7
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Re: yanmar 2qm20 - rpm question

If you can only get 2300 out of gear with new injectors check the exhaust for blockage.
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Old 03-07-2012, 06:48   #8
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Re: yanmar 2qm20 - rpm question

yeah - about 1900 under load, 2300 unloaded, i read somewhere these eng will run up to about 2900, but that comment about the prop sounds a good explanation - its not a problem though, i just wondered. I'm guessing the injectors date with the engine which probably explains the missing revs, be interesting to see what happens when i put in the new nozzles, but it probably wont make any functional difference - its still gonna make hull speed at 1600
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Old 03-07-2012, 19:26   #9
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Re: yanmar 2qm20 - rpm question

sorry I read it as 2GM20
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