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Old 01-11-2021, 08:24 AM   #1
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Idle RPM.

Just looking to see what other Diesel engines are set at for idle. New to me Tollycraft with twin Mtu 8v183 te92 mechanical injection, port motor 650 rpm starboard 700 rpm. I am looking to even them out, up or down.
Thanks and what a great fourm.
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Old 01-11-2021, 08:42 AM   #2
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Just looking to see what other Diesel engines are set at for idle. New to me Tollycraft with twin Mtu 8v183 te92 mechanical injection, port motor 650 rpm starboard 700 rpm. I am looking to even them out, up or down.
Thanks and what a great fourm.
How are you reading the rpms? Instruments?

Do you have syncs on your controls?
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Old 01-13-2021, 08:17 AM   #3
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Slow idle is fine but there is a caution.

Sometime you will shift from one direction rapidly to the other direction.

This should not cause the engine to stall from the load of changing the rotational mass of the drive train to spin in the opposite direction.
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Old 01-13-2021, 08:37 AM   #4
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Slow idle is fine but there is a caution.

Sometime you will shift from one direction rapidly to the other direction.

This should not cause the engine to stall from the load of changing the rotational mass of the drive train to spin in the opposite direction.
He is trying to match the idle RPMs he is reading.
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Old 01-13-2021, 08:52 AM   #5
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Have your tachometers calibrated first.

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Old 01-13-2021, 08:54 AM   #6
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Try swapping the feeds -- wires or mechanical cables -- between the two engines. I'll bet some of it is just instrument error. The two tachs (engine room and wheelhouse) on our single Cat 3406 typically differ by 20 or 30 rpm.


Or, buy a non-contact tach on eBay for $15 or so. The cheap ones are not very accurate, but they are consistent, so you could easily compare the two engines.


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Old 01-13-2021, 08:56 AM   #7
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Part of the question was should he adjust one engine idle UP or the other DOWN. FF is correct that if idle is to slow the engine may stall if a quick shift is done.

I started this yesterday and will finish.

What are your tachs? If they are typical electrics, NOT digital, then before deciding what to do go to E Bay or Amazon and get a handheld laser pointer digital phototach for about $20 and use it to check you dahs tach accuracy. THese digitals are accurate to +/- 2 rpm.

Your dash tachs if really good ones may be ok but I have seen , I thought good ones, off by at least 100 rpm.

Before playing with the engines find out if your existing tachs are correct.
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Old 01-13-2021, 09:49 AM   #8
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Most here think it’s cool to idle low as they then sound like a bigger heavier boat and bigger and heavier is heavily worshiped here.

Most probably idle too low. And it quickly gets to be a problem if they are overpropped as some to many are.

I set my idle high enough so when I back out of my slip and pass through neutral to get to fwd gear my engine dosn’t quit when cold. 900rpm for me but smaller engines mostly idle faster.
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Old 01-13-2021, 10:10 AM   #9
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There's likely a spec for your engine and/or gear WRT idle speed. As others posted, you need to confirm that whatever you're using to determine idle speed should be calibrated to insure that what's being indicated is indeed, correct. Verify that, then set the idle speed to what's specified in the engine/trans manual/spec. Info you obtain on a forum is worth what you paid for it.
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Old 01-13-2021, 10:21 AM   #10
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Hey Steve,
Most all people go to forums for opinions .. and they get-um.

But there are things that are better applied that are done correctly according to the correct source.

But re idle speed for a trawler what you’d get from an engine manufacturer may be best suited for a front end loader or some other industrial machine. May not be best for a boat.
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Old 01-13-2021, 10:27 AM   #11
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I don't know the 8V MTU very well, never fiddled with the idle on one. I do know that some of the German V engines the idle setscrews are under the charge air cooler, a major PITA to adjust. Not sure of the MTU.

In general I set 6 and 8 cyl engines at 600 hot idle in neut. Then try a shift from rev, quickly through n and into fwd. If it stalls, then try 650. If owner complains about it stalling cold, I'll come back and set it up a bit more. Some owners complain if it stalls, some know to pause a sec in neut to let prop coast down.

Some engines with Bosch mechanical pumps simply will not idle that low. And setting the idle involves two screws, so you need to know what you are doing. Look in the MTU manual for instructions on your specific engines.
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Old 01-13-2021, 10:40 AM   #12
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Well I guess I was just trying to answer the actual question he asked rather than one I thought he should have asked!
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Old 01-13-2021, 01:18 PM   #13
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A. Can you hear a difference in the sound between the 2 engines?
B. Get the RPM meters calibrated
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Old 01-13-2021, 08:48 PM   #14
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Your ears should tell you if they are synch'd. Measuring 50 rpm with a tach is pretty difficult. When you put both in gear do they both still run? If so, do you drive around in circles? If not, with the information provided, leave 'em.
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