1972 Quadrajet - Question about rear "port"

[Bruce L.]

I am rebuilding a 7042202 Quadrajet, correct for my 72 roadster/auto car. I notice an
uncovered "port" at the center of the rear of the main casting (venturi section) that has
two screw holes (above and below the port). There were no screws and no cover on this
port when I got the carburetor, and, after seeing many others with no covers, I suspect
that this is correct.

However, at the upper left inside corner of this large rectangular port, there is a small
square hole that leads directly to the internal section of this casting (see attached picture).
I also have a 7042203 Quadrajet which has a similar configuration, although the square
hole is irregularly shaped, and very jagged at the edges.

Is that hole supposed to be there? Everyone cautions about preventing vacuum leaks,
yet this looks like one big whopper of a leak to me.

If it is supposed to be there, what is the purpose of this hole? Also, what are the screw
holes above and below the rectangular port for?

Thanks for any advice.

[Clem Z.]

this is where the HIC,hot idle compensator

used on earlier carbs went. the screws were used to hold the cover over the HIC

[Bruce L.]

Re: this is where the HIC,hot idle compensator

Thanks Clem. Your responses are always very helpful for me.

So, a big square hole in the back of my center casting is correct?

This is a pathway for outside air right into the venturi throat.......

Bruce

[Clem Z.]

Re: this is where the HIC,hot idle compensator

are you saying that a hole runs from this recess into the venturi? that is not correct because that would allow unfiltered air into the engine

[Joe L.]

Re: this is where the HIC,hot idle compensator

Bruce-----

I think that you'll find that none of the holes will go "through". In other words, all of the holes are "blind". Any that do seem to go "through" will go to a "blind" area that you can't see with the carb assembled.

As clem mentioned, the "casting feature" that you have depicted was used for some earlier Q-Jets (but not all earlier Q-Jets) and is not used for your particular carb model. However, it is present on most Q-Jet float bowl castings.

Carbs which use this feature for the HIC usually also lack the bowl vent on the front of the top of the air horn.

[Clem Z.]

if any hole go thru into the venturi area

you have a defective casting and i would epoxy the hole closed

[Bruce L.]

Re: this is where the HIC,hot idle compensator

Clem and Joe - Thanks again for clarifying. I added a note to the attached picture to explain
the condition of my carb.

I found a diagram on the web for the HIC, and it looks like the round hole inside
the rectangular HIC port would lead to the area below the throttle plate, but, as
you can see from the picture, that hole is plugged on my carburetor.

The only "hole" inside the HIC rectangular well is that small square hole at the top
left corner. Since my carb is disassembled, I can see where it goes. It seems to lead
into the "inner regions" of the venturi casting. It may lead to a blind passage when the
carb is reassembled. I will look at it more closely tonight to see.

If anyone has access to a similar Quadrajet with no HIC, I'd appreciate knowing whether
that small square hole is unplugged or plugged.

Thanks -- Bruce

[Clem Z.]

i just checked a later model carb w/o HIC

and there is no square hole there. the recessed area is there but no square hole

[Joe L.]

Re: this is where the HIC,hot idle compensator

Bruce-----

I have many 1969 Q-Jet carbs, including at least 4 NOS examples. Some have the hole and some don't. At least one carb that I used on the car and which functioned perfectly has the hole. I don't think that the hole, present or not, makes any difference.

[Bruce L.]

Re: i just checked a later model carb w/o HIC

OK.....Clem has a later model Quadrajet with no square hole inside the HIC recessed well.

I have a 7042202 with a square hole (see the above picture).

I also have a 7042203 with a small, irregular hole in the same area of the casting.
This hole is smaller than the square hole on the 7042202, and looks suspiciously like
a thin-walled plug over the square port that has partially eroded away over the years.

I have no problem with an epoxy repair to close the hole on the 7042202, since I am
building it for a driver car. However, since I bought the carb as a correct part number
plus correct date code, this "defect" in the casting is a minor heartbreaker.

If the square hole was plugged by a thin metal part of the casting, this must have failed
on many Quadrajets, or I have been incredibly unlucky to have found 100% failures on the
two carbs I have. I wonder how the square hole affects carb operation. The 7042203 carb
is on my car now, and it runs pretty well, except for some uneveness at idle. I may try
plugging that hole on the back, and testing it to see if it has any effect.

I will be interested to hear if any others find this hole unplugged on their Quadrajets without
HIC.

Thanks for all the advice. This place has been a godsend of good information for me.

Bruce

[Clem Z.]

if the hole dead ends when the top is installed

it will cause no problems. if not epoxy it from the inside where no one will see.

[Chuck S.]

Re: this is where the HIC,hot idle compensator

Just curious...Did you take a paperclip or a short piece of wire to see if the "square hole" actually has a passage through to the inside of the carb? As Joe said, the passage may be functionally blocked by a thin web of casting metal inside where you can't see it.

It doesn't appear that the carb has ever had the HIC installed because the screw holes for the cover don't appear to have been threaded. If the passage isn't internally blocked, this would have to be a faulty casting.

[Bruce L.]

The final (?) word

Thanks to all for the help and advice. I looked the carb over closely last night
and I found that the passage from that square hole inside the HIC well leads
directly upward to a hole in the top edge of the center venturi section casting.

That hole is blocked by both the gasket and the choke plate cover when it is
installed, so, as many of you suspected it is a blind opening that won't affect
the carb operation.

Has me curious about how the configuration would work for a carb that
uses a HIC. I suspect that HIC carbs use a different choke plate that includes
a port to fresh air.

Thanks again everyone -- Bruce

[Clem Z.]

that rectangular hole does not do the venting

the round projection at the bottom of the cavity is opened to a hole in the base plate that opens into the intake manifold. a bimetalic spring allows air into this hole which leans out the idle mixture under certain hot conditions

[Clem Z.]

a very good book on Qjet carbs on how

they work and how to service them is writen by doug roe and published by HP books called "rochester carburetors." it does all rochesters not just Q jets