carb gasket help plz.

[yote1234]

I have a stock factory intake manifold "like one pictured" and the original quadrajet carb, my question is I was given 3 gaskets. a thick one a thin one and a thin metal one. so can anyone tell me if I need all three? and if so which order do they go?
h e l p ... p l e a s e

[69 MOD]

You only need the thick one with the inserts.
The inserts prevent over tightening the carb.


MOD

[hamjet]

the metal shim should be used with that type intake, otherwise the exhaust that warms the base of the carb will burn through the gasket material and cause vacuum leaks and burn fuel at the primaries before it gets to the cylinders. I bought a big block with a spread bore carb when I was a young lad and couldn't bring the timing down and the raw gas fumes burned my eyes, it ran like poo, when I pulled the carb, the exhaust had burned through the gasket and was burning the fuel. I bought the shim with gaskets and it ran like a champ....

[yote1234]

Originally Posted by hamjet

the metal shim should be used with that type intake, otherwise the exhaust that warms the base of the carb will burn through the gasket material and cause vacuum leaks and burn fuel at the secondaries before it gets to the cylinders. I bought a big block with a spread bore carb when I was a young lad and couldn't bring the timing down and the raw gas fumes burned my eyes, it ran like poo, when I pulled the carb, the exhaust had burned through the gasket and was burning the fuel. I bought the shim with gaskets and it ran like a champ....

so can you tell me which is first? im thinking the thick base gasket is the first then the metal then the thin gasket? dave

[hamjet]

I believe thats correct, its been along time, but it makes sense in that order...

[davepl]

You NEED to know what year the carb and intake are. For some, like the 1969 427/390, it's a weird setup with a metal gasket and two composite and they must go in the right order. Vettenet has a ton of info (and a supplier of the correct ones) because it's a common issue.

On my '70 402 truck you need only the thick one with the "anti overtightening" inserts as mentioned. But if you had a carb and intake from a '69 you could wind up with a massive vacuum leak as it requires the right gasket stack to cover some open paths.

Sorry, I know this only makes it more confusing for you.

[yote1234]

Originally Posted by davepl

You NEED to know what year the carb and intake are. For some, like the 1969 427/390, it's a weird setup with a metal gasket and two composite and they must go in the right order. Vettenet has a ton of info (and a supplier of the correct ones) because it's a common issue.

On my '70 402 truck you need only the thick one with the "anti overtightening" inserts as mentioned. But if you had a carb and intake from a '69 you could wind up with a massive vacuum leak as it requires the right gasket stack to cover some open paths.

Sorry, I know this only makes it more confusing for you.

the intake and carb are feb of 1971. the carb I bought nos and the intake came off a 71 c10 w/402. hope this helps?

[425HP409]

You "do not" need the steel shim gasket, it is only used with the earlier intakes that have the carburetor exhaust crossover holes that are drilled from the carburetor to the exhaust passages. See photo for the ones that use the steel shim. Only use the thick one with the inserts with your intake.

[Ol Blue K20]

I've always used the thick one and only the thick one and had no issues

[yote1234]

Originally Posted by 425HP409

You "do not" need the steel shim gasket, it is only used with the earlier intakes that have the carburetor exhaust crossover holes that are drilled from the carburetor to the exhaust passages. See photo for the ones that use the steel shim. Only use the thick one with the inserts with your intake.

im wondering if the slot cut through to the secondaries is the same as having the two holes?

[kenn]

Hello
No they are not . Some are opened up at the ends and some are not. As stated if the exhaust is opened to the carb base you need the metal one.

[yote1234]

can anyone else add to this?
3=no
1=yes

[425HP409]

Originally Posted by yote1234

im wondering if the slot cut through to the secondaries is the same as having the two holes?

As stated, you only need the steel shim gasket if the intake has the holes drilled through to the exhaust like the picture I posted. The holes are at the ends of the slot in front of the primaries. You ONLY need the thick gasket.

[davepl]

Originally Posted by 425HP409

As stated, you only need the steel shim gasket if the intake has the holes drilled through to the exhaust like the picture I posted. The holes are at the ends of the slot in front of the primaries. You ONLY need the thick gasket.

You are probably right, but I seem to recall there were also issues where vacuum could leak between the throttle bores and the carb body if the right gasket wasn't used - but only on some models of the Q-jet. Hence the need in some applications of a gasket that just has the holes for the throttle bores.

But if the heat crossover isn't drilled, odds are he just needs the fat gasket as you say.

You know what's probably a real ***** to diagnose? Someone with the wrong gasket on an engine where the heat crossover has been blocked at the head so that it's not a huge leak, but a vacuum leak nonetheless.

[yote1234]

well when the engine builder performed the brake-in by running the engine on a stand for 20 min, he did say there was a vacuum problem. and he used only the thick gasket. and he said when he plugged the vacuum line from any of the carb ports to the distributer it added 10 degrees to the timing. so?..... is it a problem in dist?
any help would be appreciated. dave

[davepl]

Honestly I would take the manifold part number and figure out what the application was and then look up that year and model. I'm happy to assist, I have parts books for the '69 Impala and for our trucks.

What you describe with the distributor sounds like a vacuum leak, and here's my theory as to why:

- At idle there should be no vacuum advance and connecting the correct ported vacuum to the dist should cause no advance. Manifold vacuum would, but not the carb ported vacuum when hooked up correctly.

- With a vacuum leak they probably had to open the throttle blades wide enough at idle that it's uncoveriring the transfer slot (or whatever its called, I'm an EFI guy) thereby causing vacuum to be present at the ported vacuum port.

425HP409 has a photo above of the one that absolutely needs the shim gasket. You can see the holes at the outside end of the slot up front, and that goes right to air down at the exhaust head riser.

What I do not know is whether some Q-Jets require a full gasket such that otherwise they'd leak at the body.

I think you're fine with the fat single gasket, but would need to know the original application of the manifold and carb for me to feel right saying either way.

A local shop wound up drilling holes through my primary blades to get a stronger signal, and I don't think its needed for a ZZ502. I probably have a vacuum leak myself, but I think mine is on the underside of the intake (a whole separate story).

[yote1234]

Originally Posted by davepl

Honestly I would take the manifold part number and figure out what the application was and then look up that year and model. I'm happy to assist, I have parts books for the '69 Impala and for our trucks.

What you describe with the distributor sounds like a vacuum leak, and here's my theory as to why:

- At idle there should be no vacuum advance and connecting the correct ported vacuum to the dist should cause no advance. Manifold vacuum would, but not the carb ported vacuum when hooked up correctly.

- With a vacuum leak they probably had to open the throttle blades wide enough at idle that it's uncoveriring the transfer slot (or whatever its called, I'm an EFI guy) thereby causing vacuum to be present at the ported vacuum port.

425HP409 has a photo above of the one that absolutely needs the shim gasket. You can see the holes at the outside end of the slot up front, and that goes right to air down at the exhaust head riser.

What I do not know is whether some Q-Jets require a full gasket such that otherwise they'd leak at the body.

I think you're fine with the fat single gasket, but would need to know the original application of the manifold and carb for me to feel right saying either way.

A local shop wound up drilling holes through my primary blades to get a stronger signal, and I don't think its needed for a ZZ502. I probably have a vacuum leak myself, but I think mine is on the underside of the intake (a whole separate story).

hi davepl. I would appreciate any help you can give me on looking up the numbers for correct gaskets. my intake # is 3855287 just like the one pictured here. from what I hear is if im getting full vac at idle then I am pulling vac straight from intake and not carb although it is plugged into the carb? and with a fresh rebuild im thinking gaskets? dave

[yote1234]

heres the carb I have #7041209 quadrajet