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Author Topic: Detroit 6-71 ilding  (Read 7243 times)
thejumpsuitman
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« on: October 21, 2009, 10:40:26 AM »

Is it really normal for the DD 6-71 to idle up and down?  Does it have something to do with keeping oil pressure up?  Every bus I have looked at with the 6-71 idles that way started up...  One mechanic I talked with said it is how they are supposed to idle, but wanted to hear what you guys had to say about it.

Thanks
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buswarrior
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« Reply #1 on: October 21, 2009, 11:42:51 AM »

Do you mean the Detroit Diesel "loping idle"?

happy coaching!
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thejumpsuitman
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« Reply #2 on: October 21, 2009, 11:54:43 AM »

Do you mean the Detroit Diesel "loping idle"?

YES!  Well put...  Loping idle.  What is that?

happy coaching!
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Just Dallas
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« Reply #3 on: October 21, 2009, 12:02:15 PM »

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« Reply #4 on: October 21, 2009, 07:11:08 PM »

I would guess that is normal, simply because mine does it when first started but smooths out once it is warmed up.

Most non-electronic engines don't idle as well cold as when warm.
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bobofthenorth
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« Reply #5 on: October 21, 2009, 07:48:59 PM »

I dunno why they do that but they don't have to.  Mine idles dead smooth hot or cold. 
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John Z
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« Reply #6 on: October 21, 2009, 11:12:44 PM »

I was told by an old mechanic that it was normal and right, and he told me "Don't let anyone touch that engine. If they don't lope when they are cold, they are set too rich." Besides, it just sounds right to me, and i would hate to lose that sound! BTW, it idles perfectly smooth once warmed up.
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« Reply #7 on: October 21, 2009, 11:44:01 PM »

First off-there is no such thing as a Diesel engine running "rich"-unless it has a plugged air cleaner and belching out black smoke-then that's running rich.
I have a turbocharged, intercooled 8V-71 with 9G75 injectors, that when it starts, no matter how cold it is, it NEVER lopes, nor should it ever. If it does, you just have a mechanic that doesn't know what he is doing.  My bus is the third Detroit 2 stroke vehicle I've owned, and none of them ever loped on start up.  All were mechanically operated, and tuned every year.  The first two were tuned at Delaney and Ahlf in Bakersfield, and my bus with Don Fairchild, also in Bakersfield (Don also worked with D and A).
Do yourself a favor and find a competent mechanic that will properly tune your engine to run smoothly all the time.  Good Luck, TomC
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« Reply #8 on: October 22, 2009, 04:54:38 AM »

so what exactly causes the lope? I understand it's a tuning issue but what aspect of the tuning?  BTW mine lopes for a few minutes as well and then settles done, runs purty and doesn't burn oil or run hot, so other than checking the injector clearances I am hard pressed to go messing with the governor.
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johns4104s
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« Reply #9 on: October 22, 2009, 05:01:05 AM »

I have three 4104,s They all Lope until they warm up.

John
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bevans6
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« Reply #10 on: October 22, 2009, 05:51:04 AM »

8V-71, lopes for  15 seconds when i start it under 50 degrees, other than that it's smooth.   I wonder if the idle is set too high, but I don't have a tach.  How do you adjust idle speed anyway?  I had a thought that it would be easier to engage first gear if the idle where set lower (actually the other way round, I thought that it would be harder to engage first gear if the idle were set too high...)

I think the loping has to be down to inconsistent combustion, which would point to over-fueling for the condition at hand, I would think.  When the combustion chambers are dead cold, they would tend to incompletely combust, would they not?

Brian
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« Reply #11 on: October 22, 2009, 05:58:02 AM »

I guess I'll have to pay attention to my engine at idle to see if it lopes...
I do know that the idle of the engine will decrease and increase when the compressor cuts in and out. just a different load on the engine.

Correct me if I'm wrong but lope is just the engine coasting/slowing down between power strokes. You hear it in Harley motorcycles because the two cylinders fire very close to each other and then there is a long time before they fire again.

Most multiple cylinder engines are balanced in there firing order. For every power stroke of a cylinder there is a cylinder that will have a power stroke just opposite. A two cylinder engine with opposite power strokes will still lope at a slow idle because there is more time between power strokes, but the lope will seem balanced. The more cylinders you have the more overlap of power strokes and the smoother the engine will run.


.
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« Reply #12 on: October 22, 2009, 06:21:42 AM »

The loping is in the governor, as Dallas said, the buffer adjustment.  I don't think it's a big deal to leave it that way unless you are doing a tune up anyway.

If everything is adjusted correctly, there should be no change when the air compressor kicks in.  It may sound different but the RPM's should remain the same.

Incorrect buffer and low idle speed will also affect the ability to do a dead throttle start.
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« Reply #13 on: October 22, 2009, 06:24:52 AM »

Probably needs new plug wires lol.
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bevans6
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« Reply #14 on: October 22, 2009, 06:48:33 AM »

Maybe it's good to define a term or two...   Shocked

what I mean by a loping idle in a Detoit is the idle speed surges up and down maybe 50 - 100 rpm on a semi-regular basis, maybe every 5 seconds or so.  A pulsing change in idle speed, that's pretty regular.

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
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