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Intake manifold Crack [message #304393]

Thu, 28 July 2016 00:28

Greenchevelless

Messages: 25
Registered: February 2013
Location: Tulsa, Okiehoma

Karma: 0

Junior Member

So I have my intake off to install new gaskets and the crossover blockoff plates. Theres a bunch of carbon deposits on the floor of the intake under the carb and I can see a crack on the wall between the secondary ports. I do not see a crack on the bottom of the crossover. Can I just get away with the block off plates? Does the carbon buildup mean that it has been leaking exhaust into the intake?

1976 Palm Beach 26'

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Re: [GMCnet] Intake manifold Crack [message #304399 is a reply to message #304393]

Thu, 28 July 2016 06:24

Kingsley Coach

Messages: 2691
Registered: March 2009
Location: Nova Scotia Canada

Karma: -34

Senior Member

Do you have a picture? Tks

Mike in NS

On Thu, Jul 28, 2016 at 2:28 AM, Dan wrote:

> So I have my intake off to install new gaskets and the crossover blockoff
> plates. Theres a bunch of carbon deposits on the floor of the intake under
> the carb and I can see a crack on the wall between the secondary ports. I
> do not see a crack on the bottom of the crossover. Can I just get away with
> the block off plates? Does the carbon buildup mean that it has been
> leaking exhaust into the intake?
> --
> 1976 Palm Beach 26'
>
> _______________________________________________
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> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Michael Beaton
1977 Kingsley 26-11
1977 Eleganza II 26-3
Antigonish, NS

Life is too short to hold a grudge; slash some tires and call it even !
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Re: Intake manifold Crack [message #304400 is a reply to message #304393]

Thu, 28 July 2016 07:15

Matt Colie

Messages: 8547
Registered: March 2007
Location: S.E. Michigan

Karma: 7

Senior Member

Greenchevelless wrote on Thu, 28 July 2016 01:28

So I have my intake off to install new gaskets and the crossover blockoff plates. Theres a bunch of carbon deposits on the floor of the intake under the carb and I can see a crack on the wall between the secondary ports. I do not see a crack on the bottom of the crossover. Can I just get away with the block off plates? Does the carbon buildup mean that it has been leaking exhaust into the intake?

While my experience with old 455s is limited, I have seen this in other carbureted V form engines. It forms when liquid fuel drips on the hot crossover after shutdown. There does not have to be any crack there for it to accumulate.

So, if you scrape that carbon off and can see a crack, don't worry about it...

Someone else may have better and more direct experience.

Matt

Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit

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Re: [GMCnet] Intake manifold Crack [message #304413 is a reply to message #304393]

Thu, 28 July 2016 09:50

USAussie

Messages: 15912
Registered: July 2007
Location: Sydney, Australia

Karma: 6

Senior Member

G'day,

After you install the block off plates the crossover will run temps that follow the oil vapor temps in the lifter valley.

I have seen a photograph of an Infrared Temp Gage reading 170° F shot through the choke hole in the manifold after block off plates
were installed. Normally that area will run over 1100° F.

I would carefully clean crack on both the lifter valley side and bottom of the carb plenum (if there is one) then smear red RTV
(hi-temp) into both.

Regards,
Rob M.
USAussie - Downunder
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426

-----Original Message-----
From: Gmclist [mailto:gmclist-bounces@list.gmcnet.org] On Behalf Of Dan
Sent: Thursday, July 28, 2016 3:29 PM
To: gmclist@list.gmcnet.org
Subject: [GMCnet] Intake manifold Crack

So I have my intake off to install new gaskets and the crossover blockoff plates. Theres a bunch of carbon deposits on the floor of
the intake under the carb and I can see a crack on the wall between the secondary ports. I do not see a crack on the bottom of the
crossover. Can I just get away with the block off plates? Does the carbon buildup mean that it has been leaking exhaust into the
intake?

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Regards, Rob M. (USAussie) The Pedantic Mechanic Sydney, Australia '75 Avion - AUS - The Blue Streak TZE365V100428 '75 Avion - USA - Double Trouble TZE365V100426

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Re: Intake manifold Crack [message #304415 is a reply to message #304393]

Thu, 28 July 2016 10:25

roy1

Messages: 2126
Registered: July 2004
Location: Minden nevada

Karma: 6

Senior Member

Cracks you mention are common with the Motorhome that has an open crossover. The high heat with an unblocked crossover contributes to carbon buildup. If I were you I would scrape off the carbon add your block off plates and put it back together.

Roy Keen Minden,NV 76 X Glenbrook

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Re: Intake manifold Crack [message #304416 is a reply to message #304393]

Thu, 28 July 2016 10:34

Greenchevelless

Messages: 25
Registered: February 2013
Location: Tulsa, Okiehoma

Karma: 0

Junior Member

Well I have the intake soaking in purple power degreaser at the moment. Here in a few hours I am going to stick it in the wash cabinet and hope that it gets most of it out. I can then see the extent of the cracks. I do not see any on the underside of the manifold that go into the lifter valley.

1976 Palm Beach 26'

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Re: [GMCnet] Intake manifold Crack [message #304420 is a reply to message #304416]

Thu, 28 July 2016 10:46

James Hupy

Messages: 6806
Registered: May 2010

Karma: -62

Senior Member

If you can look down the carb passage and see cracks, then your problem is
more serious. They will open up as the engine warms up, and leak vacuum.
Sometimes crossover exhaust will ignite incoming fuel/air causing popping
and backfiring. Blocking the crossovers will stop the popping, but not the
vacuum leaks. Cracked cast iron will not heal itself.
Jim Hupy
Salem, Oregon
78 GMC ROYALE 403
On Jul 28, 2016 8:35 AM, "Dan" wrote:

> Well I have the intake soaking in purple power degreaser at the moment.
> Here in a few hours I am going to stick it in the wash cabinet and hope that
> it gets most of it out. I can then see the extent of the cracks. I do not
> see any on the underside of the manifold that go into the lifter valley.
> --
> 1976 Palm Beach 26'
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
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Re: Intake manifold Crack [message #304455 is a reply to message #304393]

Thu, 28 July 2016 17:17

Greenchevelless

Messages: 25
Registered: February 2013
Location: Tulsa, Okiehoma

Karma: 0

Junior Member

But if its cracked from the runners into the crossover then with the blockoff plates and mr gasket 404 gaskets, it should be sealed as long as its not cracked into the lifter valley. Am i correct? So when it heats up it will only pull a vacuum on a sealed chamber instead of leaking vacuum or sucking exhaust (which is now blocked off) I would smear rtv over the cracks in the runners but im afraid if it comes loose it will damage a valve.

1976 Palm Beach 26'

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Re: Intake manifold Crack [message #304569 is a reply to message #304393]

Fri, 29 July 2016 22:40

Greenchevelless

Messages: 25
Registered: February 2013
Location: Tulsa, Okiehoma

Karma: 0

Junior Member

Guess im reinstalling it with the block off plates and going on with life.

1976 Palm Beach 26'

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Re: Intake manifold Crack [message #304584 is a reply to message #304569]

Sat, 30 July 2016 12:05

wally

Messages: 643
Registered: August 2004
Location: Omaha Nebraska

Karma: 5

Senior Member

Greenchevelless wrote on Fri, 29 July 2016 22:40

Guess im reinstalling it with the block off plates and going on with life.

We made plates that fit over the crossover ports and pressurized with air to check for cracks.
http://www.gmcmhphotos.com/photos/power-level-valve-fitting/p18121.html
My present intake crossover is filled with high temp silicone and has block plates. It leak checked OK before. I didn't want to do the Zamak or aluminium melted metal fill.
JWID

Wally Anderson
Omaha NE
75 Glenbrook

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