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
