Q-jet

N

nate chase

New member
Apr 26, 1999
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Ok you carb fans... here's a question. I had my air cleaner off and drove
around (highway) and was wondering the following.... My q-jet back two
barrels rarely seem to kick in. Even under FULL throttle they barely open
up. Seems to be operated by vacuum linkage w/o an adjustment on the linkage
itself. Is there someway to get them to kick in quicker? I think I'm running
mainly on the front barrels (which I realize is good) but it should HOWL
when you really floor it (I would think...seems to a little but they just
don't REALLY open up). I'll read the book shortly but was just wondering if
those back to jugs ever REALLY open up or even if the should?

Nate '75GB (Throttled but not floored yet) Omaha
 
Hi Nate,
I believe there's a cold lockout I think on rt. side butterfly stems/rods, I
think it works with choke. There is also a set screw/allen on air horn to
adjust spring tension for opening of air horn( this top valve gets sucked
opened). Don't have spec. handy for air horn adjust. Hope this helps
alittle.
Ron

- ----- Original Message -----
From: Nate Chase
To: GMC
Sent: Sunday, November 07, 1999 12:27 PM
Subject: GMC: Q-jet

> Ok you carb fans... here's a question. I had my air cleaner off and drove
> around (highway) and was wondering the following.... My q-jet back two
> barrels rarely seem to kick in. Even under FULL throttle they barely open
> up. Seems to be operated by vacuum linkage w/o an adjustment on the
linkage
> itself. Is there someway to get them to kick in quicker? I think I'm
running
> mainly on the front barrels (which I realize is good) but it should HOWL
> when you really floor it (I would think...seems to a little but they just
> don't REALLY open up). I'll read the book shortly but was just wondering
if
> those back to jugs ever REALLY open up or even if the should?
>
> Nate '75GB (Throttled but not floored yet) Omaha
>
>
>
 
Hi again Nate,
Just remembered air horn spring tension, When you loosen the set screw, air
horn should open slightly then start increasing tension and when air valve
just starts to hit stop tighten screw another 1/2to3/4 turn then tighten
lock screw. and it should be good, too loose and it'll bog. Choke adjust by
bending linkage, should be a "u" shape somewhere along its length for the
adjustment per specs.

Ron

- ----- Original Message -----
From: Ron & Gina
To:
Sent: Sunday, November 07, 1999 12:41 PM
Subject: Re: GMC: Q-jet

> Hi Nate,
> I believe there's a cold lockout I think on rt. side butterfly stems/rods,
I
> think it works with choke. There is also a set screw/allen on air horn to
> adjust spring tension for opening of air horn( this top valve gets sucked
> opened). Don't have spec. handy for air horn adjust. Hope this helps
> alittle.
> Ron
>
> ----- Original Message -----
> From: Nate Chase
> To: GMC
> Sent: Sunday, November 07, 1999 12:27 PM
> Subject: GMC: Q-jet
>
>
> > Ok you carb fans... here's a question. I had my air cleaner off and
drove
> > around (highway) and was wondering the following.... My q-jet back two
> > barrels rarely seem to kick in. Even under FULL throttle they barely
open
> > up. Seems to be operated by vacuum linkage w/o an adjustment on the
> linkage
> > itself. Is there someway to get them to kick in quicker? I think I'm
> running
> > mainly on the front barrels (which I realize is good) but it should HOWL
> > when you really floor it (I would think...seems to a little but they
just
> > don't REALLY open up). I'll read the book shortly but was just wondering
> if
> > those back to jugs ever REALLY open up or even if the should?
> >
> > Nate '75GB (Throttled but not floored yet) Omaha
> >
> >
> >
>
>
 
Hi Nate;

Hope everything is fine with your sweatlodge. Met someone in Santa Fe
who saw your rig at Forrest City and they said it was real nice.

The rear secondaries don't open (if set properly) all the way as you
would have a loss of air velocity at low speed. Fuel would drop out of
suspension and the engine would stumble. When those bad boys open up
all the way, you have your maximum air flow. The cubic feet needed is
much lower probably at the speed you are looking at. Carbs need a
certain amount of turbulence to properly mix the fuel & air to get the
liquid gas emulsion to turn into a gas.

As a user of low axel ratio's in my cars, I battle these constantly to
keep them kicking in at the optimal point. Too early, and the engine
will stumble, not enough and you won't get the max possible out of the
engine, Hope this answers your question. walter bright, 76 GB, Delanco,
NJ.

>
> Ok you carb fans... here's a question. I had my air cleaner off and drove
> around (highway) and was wondering the following.... My q-jet back two
> barrels rarely seem to kick in. Even under FULL throttle they barely open
> up. Seems to be operated by vacuum linkage w/o an adjustment on the linkage
> itself. Is there someway to get them to kick in quicker? I think I'm running
> mainly on the front barrels (which I realize is good) but it should HOWL
> when you really floor it (I would think...seems to a little but they just
> don't REALLY open up). I'll read the book shortly but was just wondering if
> those back to jugs ever REALLY open up or even if the should?
>
> Nate '75GB (Throttled but not floored yet) Omaha
 
Nate
What you are seeing when you "punch it" are the spring loaded upper air
vanes. The acutal rear two "barrells" are directly connected to the primary
linkage and will open when you have opened the primary throttles beyond a
certain point ( near full throttle). The upper plates that you can see
moving are acutally opened by air rushing into the venturi (and aerodynamic
counter-balance). You can adust the small spring on the end of the upper
air vane shaft by loosening it off (allen wrench), and then tightening it
to about 3/4 turn after the spring pawl contacts the pin in the air vane
shaft. That will provide a bit faster response. Too fast a response can
also be bad, so make small changes and test it. Also, look down into the
actual secondaries while holding the upper plates open (engine off) and make
sure the lower throttle plates are perpendicular or a little beyond with the
throttle lever fully depressed. Often, they don't even open fully when new.
Having them open a little beyond verticle causes a little better
distribution to the rear of the manifold (according to some engine
experts....I don't run mine at wide open throttle much). That adjustment is
made by bending a small tab where the secondaries are controlled by the
primary linkage at the throttle cable. There are a number of simple and
effective changes and adjustments you can make to your carb. Doug Roe has
an excellent book on Quadrajets available at most major book stores. The
Quadrajet, in my opinion, is the most efficient automotive carb available,
and with reasonable care, will last for years (as many already have).

Gordon & Mary
77 Royale
 
>
> Nate
> What you are seeing when you "punch it" are the spring loaded upper air
> vanes. The upper plates that you can see
> moving are acutally opened by air rushing into the venturi (and aerodynamic
> counter-balance). You can adust the small spring on the end of the upper
> air vane shaft by loosening it off (allen wrench), and then tightening it
> to about 3/4 turn after the spring pawl contacts the pin in the air vane
> shaft.

> Nate, I would check to see if Jim Bounds didnot already adjust this. If you do adjust it, I would be very careful. The secondary air vane, butterfly, or whatever you want to call it springs that controls the opening of that plate do not tolerate adjustment too well by novice hands & can break real easy. walter bright
 
Gordon,
Thanks for the reply. Thought I would get more but not too much interest.
Scine I have a rebuilt I would like to be sure it is optimized for it's job.
I'll see if I can find the book you mentioned.
I did hold the upper air vanes open on the back two barrels and looked
down in there as I opened it up full throttle. The butterflies in the back
barely opened about maybe an 1/8 inch or so. I think there is a major adj
needed to open them but haven't quite figured it out yet. I doubt they open
at all when the engine is off and the throttle is pegged.
I'll bet it will REALLY run good once I get this figured out. Thanks
again...

Nate
 
> I did hold the upper air vanes open on the back two barrels and looked
> down in there as I opened it up full throttle. The butterflies in the back
> barely opened about maybe an 1/8 inch or so. I think there is a major adj
> needed to open them but haven't quite figured it out yet. I doubt they
open
> at all when the engine is off and the throttle is pegged.
> I'll bet it will REALLY run good once I get this figured out. Thanks
> again...

Nate, the secondaries won't open at all if the choke isn't completely open.
There is a linkage that blocks it. Try your experiment again with the choke
held completely open.

If the secondaries don't open all the way then, check the throttle cable;
perhaps a bracket is bent. The secondaries don't even begin to open until
the primaries are about 2/3 open, then come on quite suddenly so that they
are all the way open at the same time that the primaries are all the way
open.

Travis
 
>Gordon,
> Thanks for the reply. Thought I would get more but not too much interest.
>Scine I have a rebuilt I would like to be sure it is optimized for it's job.
>I'll see if I can find the book you mentioned.
> I did hold the upper air vanes open on the back two barrels and looked
>down in there as I opened it up full throttle. The butterflies in the back
>barely opened about maybe an 1/8 inch or so. I think there is a major adj
>needed to open them but haven't quite figured it out yet. I doubt they open
>at all when the engine is off and the throttle is pegged.
> I'll bet it will REALLY run good once I get this figured out. Thanks
>again...
>
>Nate
There are ( were? ) two varieties of Q-jets. one used mechanical opening
for the
secondaries and the other was a vacuum opening. Unless things are really
messed up bad sounds like you have the vacuum operated type. I really don't
know what is supposed to be on the MH but I would suspect is should be the
vacuum operated type.

Jim
'73 Canyon Lands
Aurora, CO & Havre, MT
 
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