427/390 carb to manifold gasket

[Gary Wilkerson]

I was looking at a BB parts interchange book which talked about the "special" gasket and tin plate used on the 948 manifold to block of exhaust gases. The 948 has a slot across the rear of the carb mounting surface.
I'm about to install another carb on this engine (correctly dated 3370).
Is there something I need to consider here?
I've never had a problem with other holley base gaskets.
What should the correct installtion consist off?

Gary

[Wayne W.]

Re: 427/390 carb to manifold gasket

Thin metal gasket goes next to the carb. Soft gasket to the manifold.

[Joe L.]

Re: 427/390 carb to manifold gasket

Gary-----

If I were you, the first thing I'd do is to eliminate that carb "pre-heat groove" in the manifold. You can functionally eliminate it by plugging the holes at either end of the groove. You can do this with drive-in, soft plugs (like miniature block "freeze" plugs) or by drilling and tapping the holes for NPT internal hex head pipe plugs. With the manifold on the engine, the drive-in plugs are the better and easier way to go.

You especially want to do the above if you want your fresh, original carburetor to lead a long and happy life. The carb "pre-heat groove" is one of the major destroyers of carburetors. Even with the stainless steel baffle plate installed correctly, there is still too much heat transferred to the carb. GM eliminated this "feature" from all manifolds after 1969.

You'll still want to install the baffle and gasket correctly as Wayne described for purposes of originality (it can be seen) as well as proper functioning of the divorced choke (i.e. the original choke rod length is designed to accomodate the exact thickness represented by the "stack height" of the gasket and baffle plate).

[Gary Wilkerson]

Re: 427/390 carb to manifold gasket

Joe,

Are these drive in plugs brass? I've seen freeze pugs smaller than what would normally be in an engine but, none that small.

Also, whats the purpose of the groove if a divorced choke is used? EGR?

Gary

[Jim T.]

Re: 427/390 carb to manifold gasket

Joe,
Since GM eliminated the open area on intakes after 69, and mentioning that carburetor will lead a long and happy life with the preheat groove plugged, do you think that the QJet was more likely to have a shorter life than a Holley?
What years had the open pre heat groove in the intake manifolds?
Do you have the size of the small plugs available?

[Terry F.]

Re: 427/390 carb to manifold gasket

On one occasion I removed crushed aluminum foil from the holes of an intake manifold. It was smashed in there real good and seemed to be working well. On another occasion I removed two little lead balls (probably fishing weights). Though they sound primative, they seemed to be working well.

If anyone wants to see a picture of my crossmember tool, let me know, I would be glad to send them one. It is finally finished.

Best regards, Terry

[Joe L.]

Re: 427/390 carb to manifold gasket

Gary-----

The small plugs may be brass or steel. They are commonly available in good auto parts stores. They are mainly used for plugging engine block oil galleries, but they have other uses, too.

The groove is for carb "pre-heat" (to speed warm up and provide better cold engine operating. Eliminating it will cause you no noticeable change, though.

[Joe L.]

Re: 427/390 carb to manifold gasket

Terry-----

I got your e-mail and thanks. I wasn't able to respond because I'm having some computer problems right now and, for some reason, can't send or respond from my @yahoo e-mail address.

[Joe L.]

Re: 427/390 carb to manifold gasket

Jim----

Yes, I think that the carb pre-heat groove was a bit harder on Q-Jets than Holley carbs. However, it's about as hard on Holley carbs, too. The configuration of the Holley makes it a LITTLE more resistant to the heat damage.

I don't recall the size of the soft plugs. I believe they are either 3/8", 7/16", or 1/2", though. All of these are commonly available.

I believe that the pre-heat groove was first used for the 1957 model year and last used for the 1969 model year. Generally, it was only used on cast iron manifolds, though. Aluminum manifolds usually didn't have it. The exception was 1968-69 L-36 manifolds. They were aluminum and did have the heat groove.

[Gary Wilkerson]

Re: 427/390 carb to manifold gasket

Thanks to all, sound advice. I'm going to plug mine

Gary

[John H.]

Re: 427/390 carb to manifold gasket

Here's the 783 iron intake from my '67 327/300 (Holley), with the passages plugged; 7/16" cup plugs worked fine on this one, but it varies with different castings. With Holleys (and no plugs), the heat can warp/crack the baseplate; with Q-Jets, the heat can cause the bowl plugs to loosen or fall out, resulting in a fire - this caused one of the biggest safety recalls in GM history in 1970 on Q-Jet-equipped cars.

[Gary Wilkerson]

Thanks John!! *NM*