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Old 05-04-2019, 03:19 PM   #1
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RPM at idle

I just pulled out my 2016 34 from winter storage and I have lots of vibration at idle. It’s idling at 600 rpm ... isn’t that too low? On the gauge it says it’s set to 800 but it’s idling at 600. When I give it throttle the rpm increase and the vibration stops ... any idea?
Cummings 425
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Old 05-04-2019, 04:02 PM   #2
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I would adjust the idle up to 800 rpm.

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Old 05-04-2019, 06:58 PM   #3
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Some engines and actually the transmission have a bit of gear rattle in neutral. Kick the idle rpmup a hundred or so. If it goes away and it isn't too high to shift into gear without a big kick, then it is fine.


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Old 05-04-2019, 07:28 PM   #4
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I would adjust the idle up to 800 rpm.

Ted
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Old 05-04-2019, 08:27 PM   #5
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I set my minimum idle at 600RPM for minimum fwd thrust, when needed, but there are extra vibes there. 800 RPM is my 'smooth' idle setting also (set by throttle). (Smooth is all relative when I've got a pair of inline 4 cyl Perkins.)
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Old 05-04-2019, 10:05 PM   #6
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My idle's RPM has less rattle than many others, but 750 works well with my four-cylinder diesel.
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Old 05-04-2019, 10:07 PM   #7
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My idle rpm is at around 400rpm.

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Old 05-04-2019, 11:07 PM   #8
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No need for low rpm idle here.
I idle the Mitsu S4L2 at 900rpm.
Very little in-and-out of gear around the marina as Iím not overpropped. It idles about 6-700rpm when cold and is quite smooth. You know itís a diesel but thereís less shake than most other engines.
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Old 05-05-2019, 06:23 AM   #9
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A Cummins 425 is probably a QSB, and that is common rail, idle is set by the computer and is normally 600. There is an option for a helm switch to bump the idle up higher, that can be installed.

If engine is shaking the boat, check the gaps under the motor mount lower rubber block, they should all be even to "float" the engine.
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Old 05-05-2019, 07:48 AM   #10
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The engine is new only has 88 hours, 2016. On the gauge I see that the rpm is "set" at 800 but right next to it,is the real reading of 600. My mechanic hasn't check it out yet just told me to take it out for a spin to see if it settles. thanks for ll the replies will keep you posted.
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Old 05-05-2019, 08:34 AM   #11
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Quote:
Originally Posted by Tigs View Post
The engine is new only has 88 hours, 2016. On the gauge I see that the rpm is "set" at 800 but right next to it,is the real reading of 600. My mechanic hasn't check it out yet just told me to take it out for a spin to see if it settles. thanks for ll the replies will keep you posted.
We have ears. We basically drive our cars while listening to the engine.
Same way with a boat. IF you hear the engine stumbling at a low RPM, jack it up 100 RPMs and listen to what is happening.
Remember the engine is cold and unloaded, warm it up and see how it idles and again, listen to the engine noise.
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Old 05-05-2019, 09:59 AM   #12
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No need for low rpm idle here.
I idle the Mitsu S4L2 at 900rpm.
Very little in-and-out of gear around the marina as Iím not overpropped. It idles about 6-700rpm when cold and is quite smooth. You know itís a diesel but thereís less shake than most other engines.
My low speed idle is at about 650 rpm. I say "about" because the Tachometers are run off the alternator, so there are setup issues. At one point I had a digital tach to check with, but since it has failed (lack of use, when I opened the case, the battery compartment was just a lump of corroded battery) I haven't replaced it. I once reset the tachs to the digital numbers, but put little stock in their present readings.

I am purposely overpropped, to achieve better fuel economy, as my boat is never pushed beyond 9 knots and spends most of its time at 8, so I run at a nominal 2000 rpm to achieve that speed. At idle, my boat speed is about 3.5 knots, so I never need to go out of gear in the marina to control speed.

No excess shake at idle, though faster is smoother.

Volvo TAMD41s 6 cyl.
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Old 05-05-2019, 11:21 AM   #13
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Keith,
You run at 2000rpm to go 8 knots?
“Nominal” meaning may not be accurate?
Do you know your engines aren’t overloaded at 9 knots? Do you ever run WOT to test engine performance?
Don’t suspect your oil is running cool but have you tested it? Or EGT ?
HaHa 3.5 knots in a marina sounds fast to me. I go in and out of gear. I shift quickly and I’m sure there’s no undo wear on the clutches .. BW gear.
What would be your top speed if you were propped to rated rpm? 10-11?
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Old 05-05-2019, 12:50 PM   #14
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My idle rpm is at around 400rpm.

L


I think my outboard cranks faster.

Suppose if the gear doesnt chatter then all could be well.
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Old 05-05-2019, 01:10 PM   #15
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Keith,
1 You run at 2000rpm to go 8 knots?
2 “Nominal” meaning may not be accurate?
3 Do you know your engines aren’t overloaded at 9 knots?
4 Do you ever run WOT to test engine performance?
5 Don’t suspect your oil is running cool but have you tested it?
6 Or EGT ?
7 HaHa 3.5 knots in a marina sounds fast to me. I go in and out of gear. I shift quickly and I’m sure there’s no undo wear on the clutches .. BW gear.
8 What would be your top speed if you were propped to rated rpm? 10-11?
1 yes
2 yes
3 yes, no black smoke, but I can make black if I go faster.
4 no, not necessary. I had a catastrophic failure due to water intrusion a few yrs ago. When torn down, no glazing, no evidence of underloading or overloading, after 1500 hrs of the same routine, so I will continue the same routine.
5 temp guages show full operating temps at 7 knots. I don't know what speed would get to full operating temp, as I haven't cruised below 7.
6 don't have one.
7 lots of room in my marina, but I feel like I am going quite slow at 3.5. Also BW gears.
8 used to go to top speed, though with lower hp engines. Never saw 10.5, hull requires lots more hp, maybe double what I have to get out of the hole. Present engines only 33% more so not nearly enough to go much faster. Going up to WOT? Won't go fast, might hurt the engines, costs in dramatically increased fuel usage, no benefit, so I never do it.
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Old 05-05-2019, 03:48 PM   #16
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Keith,
WOT every two months or so seems necessary to me. Shortly after we got Willy I ran up to WOT and after about a minute the engine died in the bottom of the Swinomish Channel. Got the Anchor ready to go but ran back and cranked on her again and she started up. Went back to the marina at low bell. So now running up to WOT for about two minutes every two months or so is standard. I post this FYI basically as your drill and methods seem to be working very well for you.

I see you’ve been around since the beginning of TF and I think you’ve had the same boat too. I checked as I had the feeling you’ve been along for the whole ride.
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Old 05-05-2019, 06:16 PM   #17
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If you fish (trolling) and the idle is smooth under power, it's really nice to be able to go maximum slow. I troll for some species slow as it will go, and bump up my idle when out of gear until it's smooth. When cold starting it's often a couple of hundred up from idle before I find smooth idling. Just a thought (if you fish).
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Old 05-05-2019, 06:53 PM   #18
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If you fish (trolling) and the idle is smooth under power, it's really nice to be able to go maximum slow. I troll for some species slow as it will go, and bump up my idle when out of gear until it's smooth. When cold starting it's often a couple of hundred up from idle before I find smooth idling. Just a thought (if you fish).
Again, my feeble memory..... Isn't there a slow speed/trolling speed adapter for some, or maybe all, brands of diesels????
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Old 05-05-2019, 08:46 PM   #19
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The engine is new only has 88 hours, 2016. On the gauge I see that the rpm is "set" at 800 but right next to it,is the real reading of 600. .
First thing is you use the term "real reading" but that's just a reading. Until checked you don't know what it's idling.

Now, with the low hours, has all the work prescribed in the manual for break in and post break in been performed? It's not at all unusual for a new engine to start idling rough and need basic adjustments. Has it been run enough at temperature and load? While running at a light load might be fine later, initially it can lead to rough performance.
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Old 05-05-2019, 08:52 PM   #20
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John Deere said my naturally-aspirated 4045 80-horsepower engine shouldn't be run at idle speed longer than five minutes. Didn't specify whether engine was under load, but I don't anticipate a problem running at idle with engine warmed and under load if engine temperature is maintained. Besides, cannot maintain "no-wake speed" if without minimal RPMs.
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