Carb-gasket-intake Q

[Wayne M.]

Photo below is of my 396 set-up: factory intake 3866963; LIST 3124 carb (but w/ 1968 service date); and the gasket Gr 3.270 # 3955527, which is called out in the AIM, and the P&A30 parts books. (Google "Dr.Rebuild" & the part # and you will see the GM engineering drawing of this gasket).

My question is: why use a full gasket for open plenum, when the base plate of the carb has 4 distinct throttle bores, and the intake manifold has two matching holes (driver side), each directing fuel/air mixture to the upper level of the divided intake plenum, while the oval bore (passenger side) directs the mixture to the lower level.

I was wondering why they didn't specify a gasket to match the manifold openings (at least), but then I see that the center plenum divider is cast (not machined) to be lower than the gasket contact surface, so any hole-matching gasket would have an internal leak to or from driver side to passenger.

Guess I'm looking for a Carb / Manifolds 101 explanation, [and I'm also probably using the wrong terms].

[Joe L.]

Re: Carb-gasket-intake Q

Photo below is of my 396 set-up: factory intake 3866963; LIST 3124 carb (but w/ 1968 service date); and the gasket Gr 3.270 # 3955527, which is called out in the AIM, and the P&A30 parts books. (Google "Dr.Rebuild" & the part # and you will see the GM engineering drawing of this gasket).

My question is: why use a full gasket for open plenum, when the base plate of the carb has 4 distinct throttle bores, and the intake manifold has two matching holes (driver side), each directing fuel/air mixture to the upper level of the divided intake plenum, while the oval bore (passenger side) directs the mixture to the lower level.

I was wondering why they didn't specify a gasket to match the manifold openings (at least), but then I see that the center plenum divider is cast (not machined) to be lower than the gasket contact surface, so any hole-matching gasket would have an internal leak to or from driver side to passenger.

Guess I'm looking for a Carb / Manifolds 101 explanation, [and I'm also probably using the wrong terms].

Wayne------


For one thing, I don't understand how gasket GM #3955527 could have been called out in the 1965 AIM. That part number did not even exist until 1969. On top of that, GM #3955527 was an intake gasket set for rectangular port heads and not a carb gasket.

The original L-78 carb gasket was GM #3868827. That was discontinued in February, 1968 and replaced by GM #3881847. The latter was discontinued in December, 1973 and replaced by GM #682914 which was discontinued without supersession about 20 years ago. I do not know how these gaskets differ.

[Wayne M.]

Re: Carb-gasket-intake Q

Sorry Joe, others. That 3955527 was a gasket "unit" (center, 2 ends, 2 exh. pipe), as you say. Carb gasket itself # 3868827 (as you say) found in Gr 3.726.

Here's the direct Doc rebuild web page link, for the original L78 gasket:

http://shop.docrebuild.com/1965CorvetteCarbGasketReplacesGM3868827.aspx

Question remains the same: would like an explanation for the gasket vs. carb base vs. intake geometry differences.

[Mark E.]

Re: Carb-gasket-intake Q

Wayne,

I can't answer your question completely, but-

- A lot of GM cars (and probably other makes too) have an open carb gasket even though the intake plenum opening isn't open.

- One contributing reason for an open gasket is to provide a vacuum signal to various ports located in bottom of the carb's base plate. I'm not sure about Holleys, but Q-Jets have ports between the primary bores that provide vacuum to the power valve and other subsystems. I've seen Q-Jet cars run like crap because an incorrect 4-hole or 3-hole gasket was obstructing these ports.

So don't underestimate the importance of installing the correct gasket, even if it doesn't "look right".