In preparation to paint my intake... I bought a new carb base gasket.
However when I took mine off. I discovered a thin stainless steel plate with a rubber gasket on each side - easily reusable.

The only real difference is that my set-up does not have the long left to right opening at the small end - depicted at the top of this pic of the new gasket. Right above the 2 small bore holes.

My guestion is..... what does this opening "do" - and what happens when it's eliminated. (note my car has run for decades with that opening blocked).

thanks! -W

 

Not sure what the opening is for off the top of my head, but a 68 stock setup would have channels in the intake manifold to warm the carb up... a system that was ultimately removed.

Are you running the stock intake? If so, you need to use that stainless steel rig or block those channels. If you're running an aftermarket intake, then that stainless plate is not needed.

 

Yes.... it's a stock intake.
It has a matching channel for the carb at that place in the stock gasket.
So you're saying that opening is to do with warming up the carb??

-W

 

The "correct" order is to put that gasket on the intake, then the steel plate, then the carb. Some people sandwich a gasket between the carb and plate.

Here's a good thread on it: http://www.yenko.net/ubbthreads/ubbt...34026&page=all

 

Ya... for whatever reason mine has a rubber gasket on each side of the steel plate. Been like that for years..... At least I know what the plate is for now.

-W

 

The slot is a heat channel, if I recall correctly. I'm thinking the AIM shows the order of assembly for the gaskets. Carburetors were installed on the engine dress line at the plant.

 

I think that channel is for distribution of manifold vacuum to various ports in the carb for internal use and for external ports.

 

The gasket shown in your photo is for the "Hot Slot" intake manifold used on ’66-’68 327/300hp, ’68 327/350hp, and ’66-’69 427/390hp Corvette engines. On this design, hot exhaust gas passes under the carb to make cold-start drivability better. The gasket in your photo can only be used on the hot slot manifold, and no other gasket can be used in its place. The gasket is part of a 2-gasket system, which consists of the gasket shown plus a stainless shield to prevent exhaust from hitting the bottom of the carb.

For a complete description, with photos, of the system, along with my recommendation of a special top gasket (with all part numbers) for use with the 2-gasket system, drop me an e-mail request for my "Q-Jet Base Gasket Kit Info" paper.

Lars
V8FastCars@msn.com

 

Here's the entire article posted with photos:

Technical Information Bulletin Rev E 1-2-12
Q-Jet Carb Base Gasket Kit Info for “Hot Slot” Manifolds
by Lars Grimsrud
©2012 Lars Grimsrud.
This article may not be published or distributed without the written permission of the author

Early Rochester QuadraJet Carburetor installations (1966-1969) often used a “hot slot” exhaust groove that runs from side-to-side in the intake manifold just forward of the primary throttle bores, popularly known as the “smiley face” manifolds. The purpose of this “smiley face hot slot” was to run hot exhaust gasses under the carb to provide fuel pre-heat on cold days. The hot gasses would heat up a stainless steel plate, and the carb was bolted directly to this stainless plate for good heat transfer. Chevy used this design on ’66-’68 327/300hp, ’68 327/350hp, and ’66-’69 427/390hp Corvette engines.

There are several problems with this arrangement:

1. A hot carb with hot fuel may run very well on cold mornings in Minnesota, but this arrangement does not run well on hot summer nights in Southern California.

2. A carb bolted directly to a steel shield is highly prone to vacuum leaks between the carb and the shield.

3. Any leak in the gasket arrangement results in exhaust gas ingestion directly into the intake manifold, which leads to poor engine performance and the possibility of engine damage if the leak becomes severe.

4. Since the “hot slot” was only used for a few years, there is a high probability of “mechanics” installing the incorrect carb base gasket (late model style), resulting in exhaust gas ingestion into the manifold and external exhaust leaks due to the incorrectly-fitting late-model base gasket.

To solve this problem, you need to use a special 3-piece carb base gasket “kit” for the “hot slot” manifolds. There are no parts stores who recognize the existence of such a gasket kit, so you have to create your own (I am no longer offering these for sale).


Here is a typical cast iron Chevy “hot slot” “smiley face” manifold. The slot was also used on the aluminum Chevy Big Block manifolds, ’66-’68 327, and on late-60’s Pontiacs. The “hot slot” is the long groove running from side to side just forward of the small primary throttle bores.


Here is the “hot slot” on the aluminum Chevy Big Block. On this manifold, you can see the exhaust gas ingestion path into the manifold due to the failure to use the correct base gasket – the bottom of the carb has been exposed directly to the exhaust gas, and this has gone through the intake manifold:


Here is the fix for the “hot slot” exhaust gas problem. In the stock configuration, you must first use a high-temperature composite gasket which lies directly on the intake manifold. Notice that it is not symmetrical – if you install it upside-down, you will get a massive exhaust leak on the driver’s side of the manifold:


Next, and placed on top of the high-temp composite gasket, is the stainless steel shield. The factory gasket “kit” ends at this point by installing the carb directly on top of this steel plate. This results in the entire bottom of the carb being heated up to the temperature of the exhaust gas, and the setup is highly prone to vacuum leaks:


To solve the problems associated with the hot carb, I add a third gasket on top of the stainless shield. This gasket has the same shape as the shield (note “bulge” on driver’s side) to prevent any exhaust leaks out the driver’s side of the manifold, and the gasket keeps the majority of the heat off the bottom of the carb:


The complete gasket kit looks like this:


This kit eliminates any exhaust leaks into the manifold as well as any external exhaust leaks. It also keeps the carb base significantly cooler than the stock setup, making the carb installation very similar to the 1971-1980 style installation (which did not use the “hot slot.”)

I am no longer offering these kits for sale, but you can build your own using the following part numbers and sources:

1. Bottom gasket (high temp with forward groove): Felpro 60043 (available from any NAPA store)

2. Middle gasket (stainless steel heat shield): Felpro 60731 (available from any NAPA store)

3. Top gasket (4-hole without groove): Mr.Gasket 4256G (available from Summit Racing)

 

Great information.

Thankyou Lars.