Author Topic: Early Chevrolet and Pontiac intakes.  (Read 3429 times)

Offline Cliff Ruggles

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Early Chevrolet and Pontiac intakes.
« on: February 14, 2012, 05:20:12 AM »
We have had a LOT of calls and emails to the shop recently from folks having MAJOR running issues when using early Chevy and Pontiac intake manifolds.

The factory used an exhaust "channel" under the carburetor on many factory intakes produced in the mid to late 1960's.

This channel was used in conjunction with a stainless steel plate to keep hot exhaust gasses out of the intake system, but still allowed them to heat the bottom of the carburetor for leaner settings and improved emissions.

It did NOT work well, and later intakes did not use this system.

In the last 6 months I've had several dozen complaints from folks trying to use these early intakes.  The problems they are having are at idle and low engine speeds, caused by not effectively sealing off the exhuast channel from the intake (internal vacuum leak).

I highly recomend plugging off the channel on both sides by driving plugs into the holes, or tapping them for plugs. 

It is also MANDITORY to use the correct gaskets under the carburetor, and the steel plate, to avoid a vacuum leak.  The gasket goes down first, then the steel plate, then the carburetor.  A second gasket can be added over the plate if desired, but the plate MUST sit on top of the correct gasket to seal off the exhaust crossover channel, or the engine will not idle well, if at all.

I've had several carburetors sent here recently from folks having this issue, where the hot exhaust gasses had actually ruined the power piston spring and were "cooking" the bottom of the carburetor as well.

With todays modern fuels, containing ethanol and whatever else they put in them to lower the boiling point, and evaporate quickly, heating up the carburetor with hot exhaust gasses just isn't going to work........Cliff

Offline troynjen@gmail.com

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Re: Early Chevrolet and Pontiac intakes.
« Reply #1 on: April 27, 2012, 08:20:14 AM »
Cliff,

I wonder if this is the problem I am running into.  I have a 1967 Pontiac Firebird 400 with the original intake manifold, and I cannot get the carb to work right.  I have tried rebuilding it several times with a kit from Napa, and even one of your kits.

In desperation, I purchased a rebuilt carb from Advanced Auto, and it has the same problem.  The engine won't idle at all, and it runs really rough.  When I stop the car, I can see fumes coming out of the carb.  Could this be the problem you are describing?

Lastly, is the passage I need to block off the one near the front of the carb?  There is a little passage in the gasket I have been using that runs underneath the front of the carb near the primary wells.  Is this what I need to plug?

Thanks!

Offline troynjen@gmail.com

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Re: Early Chevrolet and Pontiac intakes.
« Reply #2 on: April 27, 2012, 08:37:33 AM »
Cliff,

I have attached a picture of my intake manifold.  Are the passages I circled in red the ones that need to be blocked off?  On my intake, these passages are all black and sooty now, so perhaps this is my problem?

Offline Cliff Ruggles

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Re: Early Chevrolet and Pontiac intakes.
« Reply #3 on: April 30, 2012, 02:35:50 AM »
Use the correct gaskets with the stainless steel plate and it should work fine.  We recomend blocking off the holes on each end of the channel to keep the heat off the carburetor.....Cliff