intake manifold vacuum leak? - Team Camaro Tech
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post #1 of 8 (permalink) Old Sep 2nd, 14, 09:14 AM Thread Starter
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Joe
 
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intake manifold vacuum leak?

Hey guys, I just built a 400 small block with edelbrock RPM heads. The e heads were used, but appeared to be in good shape. I am using a performer 2101 manifold. Summit racing cheapie intake gaskets with only a trace of high-tack sealer on the cylinder head side. It runs fine at idle and revs nicely free, but when I put it in gear, and load the engine, it experiences intermittent popping and backfire through both the exhaust and the carb. Sometimes this is severe enough to kill the engine. It seems to get worse as the engine heats up. I am suspecting an internal vacuum leak where the intake manifold mates to the cylinder head. Vacuum at the manifold is showing 15 lbs, but drops towards 10 during a pop or backfire.

I sprayed carb cleaner all around the base of the intake, to try and find the leak, with no success.

One of the inner bolts on the intake manifold directly below the carb bolts going into the head is stripped and I could not get it to tighten. It just keeps turning. A couple others felt like they were starting to strip also, so I did not tighten them all the way to 25 ft-lbs.

I just wanted to bounce this issue off you guys and make sure I'm not going crazy.

I'm thinking I need to pull the intake off and helicoil all the holes, and reinstall the intake, with a good bead of RTV on both sides of the gasket.

What do you guys think? Does my diagnosis seem correct?

The important thing here is that you ask me what kind of car I've got. "I've got a BITCHIN' CAMARO"
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'68 350 4 speed
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post #2 of 8 (permalink) Old Sep 2nd, 14, 09:48 AM
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Don
 
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Re: intake manifold vacuum leak?

You are thinking correctly on the helicoils. place rags in lifter valley to catch all shavings, use vacumn cleaner if necessary.
Try using thicker intake gaskets, pre fit the gasket to the head and intake to check alignment. Use sealer on water passages, maybe smear on the head side gasket only?
I use RTV on the manifold ends to block, no gaskets. Allow rtv to cure and get a better fit.

Don
TC # 349
Zdld17:69 Z/RS,306, NOR141111, 9N554XXX, 12A, X3G, 59/59,723, AFR 195,CCC282/290HR, TKO 600, BU1122B1E Owner since Dec 1968

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post #3 of 8 (permalink) Old Sep 2nd, 14, 10:05 AM Thread Starter
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Joe
 
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Re: intake manifold vacuum leak?

Thanks for the reply. I did RTV only on the ends, per the edelbrock instructions.

Oh, I forgot to mention, I am using .070 thick MLS head gaskets to lower compression ratio. Do I need to run correspondingly extra thick intake gaskets?

The important thing here is that you ask me what kind of car I've got. "I've got a BITCHIN' CAMARO"
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'68 350 4 speed
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post #4 of 8 (permalink) Old Sep 2nd, 14, 10:24 AM
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Doug
 
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Re: intake manifold vacuum leak?

Use fel pro 1003 head gaskets and 1205 intake gaskets and you should not have any problem with sealing. Pull the intake back off and fix all the bolt holes with helicoils that need it. Use thin layer of permatex ultra black around the water jackets only no other sealant needed for intake gaskets. When you put everything back together make sure you have #1 cylinder at TDC. Move the balancer off "0" to about 12 degrees and install distributor.
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post #5 of 8 (permalink) Old Sep 2nd, 14, 10:31 AM
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Don
 
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Re: intake manifold vacuum leak?

Quote:
Originally Posted by kookykrispy View Post
Thanks for the reply. I did RTV only on the ends, per the edelbrock instructions.

Oh, I forgot to mention, I am using .070 thick MLS head gaskets to lower compression ratio. Do I need to run correspondingly extra thick intake gaskets?
Thicker head gaskets , will widen the intake mounting, I can see why you have a issues.
Maybe those 1205 gaskets Doug recommended will help.

Don
TC # 349
Zdld17:69 Z/RS,306, NOR141111, 9N554XXX, 12A, X3G, 59/59,723, AFR 195,CCC282/290HR, TKO 600, BU1122B1E Owner since Dec 1968

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post #6 of 8 (permalink) Old Sep 2nd, 14, 08:45 PM
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Re: intake manifold vacuum leak?

Also with gaskets that thick you will lose quench in the combustion chamber and possibly cause detonation. Study here I think recommended max is .045 including the down in the hole measurement.

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post #7 of 8 (permalink) Old Sep 2nd, 14, 09:01 PM
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Don
 
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Re: intake manifold vacuum leak?

Quote:
Originally Posted by scblucam View Post
Also with gaskets that thick you will lose quench in the combustion chamber and possibly cause detonation. Study here I think recommended max is .045 including the down in the hole measurement.
X2, You are correct. .040-045. I was thinking the same , just wondering why also. Don't know his engine combinations.

Don
TC # 349
Zdld17:69 Z/RS,306, NOR141111, 9N554XXX, 12A, X3G, 59/59,723, AFR 195,CCC282/290HR, TKO 600, BU1122B1E Owner since Dec 1968

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post #8 of 8 (permalink) Old Sep 2nd, 14, 09:15 PM
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Lynn
 
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Re: intake manifold vacuum leak?

What Scott said. You killed whatever quench you had. Worst way ever to lower compression.

How far down in the hole are the pistons?

Lynn
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