Author Topic: 8V71 Jake Brake Buffer Adjustment  (Read 489 times)

Offline richard5933

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8V71 Jake Brake Buffer Adjustment
« on: October 24, 2019, 04:44:41 PM »
When I had the bus in for its fall maintenance, the tech did a tune-up and ran through the Jakes to be sure that the settings were still good. Engine runs like a top now. One thing that I'd like to change, and I guess it's time for me to learn how to do this myself.

When he first installed the Jakes, there was a dead spot at the top of the throttle. In this small dead spot I could barely move the pedal - enough to shut off the Jakes but not enough to actually trigger the air throttle to accelerate. This was great for when I wanted to just roll down a gentle hill without the Jakes.

When he redid the adjustments this time, he got things tighter and now the dead spot is nearly gone. Now I'm either accelerating or the Jakes are on. There is no way to to just roll along anymore.

I'm assuming that there is a simple adjustment in the buffer screw that will give me back the slight dead spot. However, I also know that messing with the buffer screw incorrectly can mess up the idle (which right now is super smooth).

So, question?? Is what I'm trying to accomplish here an easy adjustment or something that I'm likely to become frustrated with? If it's relatively easy, I would appreciate some basic advice on how to accomplish this.
Richard
1974 GMC P8M4108a-125 (Custom Coach "Land Cruiser")
1964 GM PD4106-2412 (Former Bus)
Located in beautiful Wisconsin
KD9GRB

Offline buswarrior

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Re: 8V71 Jake Brake Buffer Adjustment
« Reply #1 on: October 24, 2019, 06:14:22 PM »
Turn the switch off, when you don't want jakes?

Old school in me doesn't like the idea of running default with the jakes left on. I will make the decision as to their deployment, or not.

Anyone ever induced drive wheel skid with the jakes, on slick roads? It's your habits that save you, or sink you...

Let me tell you, it is a fascinating experience, and in an old bus without an anti-lock brake system integrated...

What's your A$$ pucker reaction to the bus drifting? Brain goes into freak mode, ain't been here before, do what i've always done when $hit goes wrong... Lift throttle? Oh dear...

I'm not nearly as smart as I used to be, so remembering to change something, under certain conditions, isn't as "routine" as it used to be...

We ain't Billy Bigrigger, never mind that "faster shift" poop, we're only shifting 3 times to get into the big hole.

Leave the jake off, until you want it deployed?

Happy coaching!
Buswarrior
Frozen North, Greater Toronto Area

Offline richard5933

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Re: 8V71 Jake Brake Buffer Adjustment
« Reply #2 on: October 24, 2019, 06:44:30 PM »
Turn the switch off, when you don't want jakes?...
Happy coaching!
Buswarrior

Generally, I do this.

However, there are times when I'm in an area between needing them and not needing them, like going through a flat between grades. It's really handy to be able to turn them off by just moving the pedal through the free-play area. The way it is now, I've got to push the pedal far enough to actually start accelerating again to turn them off, but often in those situations I really don't want to add fuel, just coast. The way he's got it adjusted right now coasting is not possible - either I'm accelerating or the Jakes are on.

When he first installed them there was that little spot of free-play where I could hold the pedal and coast.

If nothing else, I'd like to have a better understanding of how the Jakes are adjusted with relation the the buffer switch. Since I've got the buffer switch with the fast idle retained, I'm having a heck of a time even finding the manual which shows the adjustment.
Richard
1974 GMC P8M4108a-125 (Custom Coach "Land Cruiser")
1964 GM PD4106-2412 (Former Bus)
Located in beautiful Wisconsin
KD9GRB

Offline luvrbus

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Re: 8V71 Jake Brake Buffer Adjustment
« Reply #3 on: October 24, 2019, 07:08:00 PM »
Screw the switch out and I mean out about a 1/4 of a inch,take a non contact rpm gun (tachometer) check the idle speed close, screw the switch back in gently till the RPM raises 25 no more than 25 rpm over idle speed with a warm engine and you are set, that information is in the DD engine manual not from me it is important to use a RPM tach gun do not depend on the tach on the dash
Life is short drink the good wine first

Offline richard5933

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Re: 8V71 Jake Brake Buffer Adjustment
« Reply #4 on: October 26, 2019, 07:30:04 AM »
Anyone able to point me to where this adjustment can be found in the manual? I've looked through the GM maintenance manual and see nothing that resembles setting the buffer. Nothing in the DD engine manual either that I can see. I've got an air throttle, fast idle, and Jakes. I'd really like to see this adjustment in a manual before attempting it.

Or, does anyone have the instructions that come with the Jake Brake buffer switch? I thought I had a copy, but it's disappeared.
Richard
1974 GMC P8M4108a-125 (Custom Coach "Land Cruiser")
1964 GM PD4106-2412 (Former Bus)
Located in beautiful Wisconsin
KD9GRB

Offline freds

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Re: 8V71 Jake Brake Buffer Adjustment
« Reply #5 on: October 26, 2019, 09:56:43 AM »
I downloaded a bunch of PDf's from there web site: www.jacobsvehiclesystems.com

I can't remember how I navigated to the documents. So I will email you the ones that I downloaded.