Home » Public Forums » GMCnet » [GMCnet] Q-Jet Replacement
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| Re: [GMCnet] Q-Jet Replacement [message #89937 is a reply to message #89934] |
Fri, 25 June 2010 21:44   |
g.winger
Messages: 792
Registered: February 2008
Location: Warrenton,Missouri
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With tuning it would probably work fine, but why would you want to??? I considor the Q-jet to be the best carb ever. You can adjust the mains quickly without removal. How fast the 2ndarys open, with how tite the spring is AND using diffrent chock pulloffs. You can adjust the primaries with a screwdriver type tool. Shoot you could adjust the primarys while you were moving down the road.(try that with a holley). With a little help of course. Q-jets are rated at 750 and 850cfm, so,,,,why would you want less power??just wondering???,,,,PL
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| Re: [GMCnet] Q-Jet Replacement [message #89943 is a reply to message #89937] |
Fri, 25 June 2010 22:14 |
jimk
Messages: 6734
Registered: July 2006
Location: Belmont, CA
Karma: 9
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There are lot of things that make the carb perform differently from other
models.
From what I know and have experienced, I would stay with the Quadra jet.
On a car,it does not matter as much , but on a MH, there are lot of things
like excess loads and wide open situations.
If you have programed a electronic fuel injection computers, you know that
there are lot you need to consider if you want it to work.
We have had great results with the stock carbs when set properly.
On Fri, Jun 25, 2010 at 7:44 PM, Paul Leavitt <leavittpaul@yahoo.com> wrote:
>
>
> With tuning it would probably work fine, but why would you want to??? I
> considor the Q-jet to be the best carb ever. You can adjust the mains
> quickly without removal. How fast the 2ndarys open, with how tite the spring
> is AND using diffrent chock pulloffs. You can adjust the primaries with a
> screwdriver type tool. Shoot you could adjust the primarys while you were
> moving down the road.(try that with a holley). With a little help of course.
> Q-jets are rated at 750 and 850cfm, so,,,,why would you want less
> power??just wondering???,,,,PL
>
> _______________________________________________
> GMCnet mailing list
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--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502
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Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
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| Re: [GMCnet] Q-Jet Replacement [message #89945 is a reply to message #89934] |
Fri, 25 June 2010 22:38 |
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I'm running a holley street avenger carb in 670 cfm with a spread bore adapter and I'm happy with it. Use 65 or 64 jets and heavy vac secondary spring. Also I use a 4 door power valve at 7,5 in of vac. Runs likd a raped ape. Will break the tires when u mash it...
73 Canyon Lands, (a.k.a. The Yellow Submarine) West Los Angeles CA
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| Re: [GMCnet] Q-Jet Replacement [message #89961 is a reply to message #89948] |
Sat, 26 June 2010 01:50 |
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Robert, last really long trip I had data for was my trip north on new years. I averaged 10.5 a gallon over approx 850 miles IIRC...
73 Canyon Lands, (a.k.a. The Yellow Submarine) West Los Angeles CA
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| Re: [GMCnet] Q-Jet Replacement [message #89987 is a reply to message #89937] |
Sat, 26 June 2010 09:59 |
tmaki
Messages: 200
Registered: September 2005
Karma: 0
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Paul Leavitt wrote:
I considor the Q-jet to be the best carb ever. You can
adjust the mains quickly without removal.
I'd be interested in that. Please detail the exact steps and
tools required to do those adjustments without removing the
carburetor - on the Rochester, not Edelbrock, Q-jet. (Yes,
you can "adjust" the idle and APT (on some) without removal.)
> You can adjust the primaries with a screwdriver type tool. Shoot you could adjust the primarys
> while you were moving down the road.
I'd be interested in that, too. Please detail the exact
steps and tools required to adjust the primaries while
moving down the road. (Yes, you can stick "a screwdriver
type tool" into the vent to push the power piston and
primary rods down into the primary jets while moving down
the road.)
In the 4"-thick file of reference material I have, including
the GM service guides for nine different Q-jets, the best
books on the subject and several hundred pages of material
from outstanding Q-jet experts, I've found no information on
adjusting the Q-jet as you describe.
> "just wondering???"
Toby "hoping for a pot of gold at the end of that rainbow" Maki
'73 Glacier 230
Riverside, CA
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| Re: [GMCnet] Q-Jet Replacement [message #89989 is a reply to message #89934] |
Sat, 26 June 2010 10:19 |
g.winger
Messages: 792
Registered: February 2008
Location: Warrenton,Missouri
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You should purchase and read at least twice Cliff Ruggles book "How to rebuild and modify quadrajets". All Q-jets have the same size mains on the secondarys. The "area" of the jet is changed by swapping rods with diffrent tapers and diameters. AND diffrent "hangers" that hold the rods on the cam. SO,,,step one. Remove air cleaner. Step two,,unscrew the 2 screws that hold the hanger on top of the cam.(between the large secondary flaps). Remove hanger. Step 3,,swap out the rods OR a hanger of diffrent height. Step four,,,,reverse 1 and 2. Its that simple. Now some have had success "carefully" bending the hanger to get diffrent height. Hpe this helps,,,but get the book and go to Cliffs website,,,Cliffs High Performance Quadrajets. Hope this helps,,,PL
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| Re: [GMCnet] Q-Jet Replacement [message #89993 is a reply to message #89989] |
Sat, 26 June 2010 11:26 |
tmaki
Messages: 200
Registered: September 2005
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Paul Leavitt wrote:
>
> You should purchase and read at least twice Cliff Ruggles book "How to rebuild and modify quadrajets".
I have it, as well as Doug Roe's book. Both thoroughly read,
margin noted, dog-eared and grease-stained. And the GMC
manual with its quite thorough chapter re: Q-jet.
> All Q-jets have the same size mains on the secondarys.
Right. Which, contrary to popular belief, can be resized,
albeit not easily.
> The "area" of the jet is changed by swapping rods with diffrent tapers and diameters.
Right. I have a collection of 17 of the different rods,
tables of all the diameters and tapers, and have an Excel
table that I set up to calculate the %-age effect rich or
lean of all combinations of both primary rods/jets and
secondary rods/hanger. I've got duplicates of CH, CV, DD,
DR, CP, CG, DP, and DH rods; M, O, P and R hangers. Need any?
> AND diffrent "hangers" that hold the rods on the cam.
Right. I have a collection of 13 different hangers starting
with D through S.
And, of course, not all those rods and hangers are of any
use in a GMC motorhome carburetor.
> SO,,,step one. Remove air cleaner. Step two,,unscrew the 2 screws
I've only ever seen one. Either a straight blade (earlier
models) or T-15 (later, and generally useless, models)
> that hold the hanger on top of the cam.(between the large secondary flaps).
> Remove hanger.
> Step 3,,swap out the rods OR a hanger of diffrent height.
> Step four,,,,reverse 1 and 2.
I've done that so many times, I can do it in my sleep. Every
time I go the junk yard, I get as many late 60's - mid 70's
Q-jets and take the hangers and rods out. If I don't already
have them in my collection, I take them. If I do, they stay
behind. (I take my inventory list with me.)
> Its that simple.
Yes, it IS that simple to change the secondary rods and
hangers. But you said you could adjust the primaries while
going down the road and without removing the carburetor.
How do you do that?
As an aside, since the secondary rod taper and the secondary
air valve opening angle are intimately related, there is
hardly any mention in the recipe esoterica about the various
secondary air valve shafts that are available. I have found
one source that goes into that detail, and since I now have
a 7043254 to dissect and examine, some of that mystery may
be solved.
At this point, I have no complaints about my GMC's
carburetor. I put in 73/53 primary jet/rod giving me 5% rich
on that side, and DG/K secondary rod/hanger giving me 2.6%
rich secondary (California 10% corn syrup, you know). The DG
is the closest I could find to the original CJ (which by the
way are still available). Also, for anybody who's looking,
the AX and BB are identical to the CJ. BB probably not too
easy to find. AX can be found in GMC/Chevrolet P-30 chassis
carbs from about '75 to '79.
Hey, the hunt is always more exciting than the kill.
Toby Maki
'73 Glacier 230
Riverside, CA
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| Re: [GMCnet] Q-Jet Replacement [message #89999 is a reply to message #89993] |
Sat, 26 June 2010 12:26 |
spiffycar
Messages: 133
Registered: May 2010
Location: Brook Park ,Ohio
Karma: 1
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The primary metering rod heights are adjustable on some Qjets depending on which throttle base plate you have. The adjuster is located between the mixture screws just underneath the center fuel inlet. It is caped off from the factory and must be drilled out to gain access to the adjuster screw. The tapered screw works by pushing against a small cam in the base plate. This cam controls the height of the power piston/primary needles in their full down position (IE high vacc./cruise mode).Since the needles are tapered this affects the amount of fuel metered through the jets. If you have ever worked with Su/Stromberg type carbs with adjustable needles then you already have an idea how this works. Raise the needles = richer cruise mix,lower the needles = leaner cruise mix. You would have to remove the carb to gain initial access to it but may be able to adjust it with on of those small "finger" screwdrivers after its back on.
Would be nice if it was an allen key head on the screw. I suppose you could probably make one for it by grinding a long taper into a long allen headed bolt that was the right thread to fit in the slot. Might be a nice adjustment to be able to get to for those that frequently go through major altitude changes in their travels.
Paul W L
76 Daytona Beach! ( EX-Palm Beach )
Cleveland,OH
& Current Card Carrying Pull A Part VIP Member
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| Re: [GMCnet] Q-Jet Replacement [message #90002 is a reply to message #89999] |
Sat, 26 June 2010 13:25 |
tmaki
Messages: 200
Registered: September 2005
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Paul W Lehmann wrote:
>
> The primary metering rod heights are adjustable on some Qjets depending on which throttle base
> plate you have.
Right. That's the APT (Adjustable Part Throttle), and the
adjustment range - to my understanding - falls into the "to
some extent" category.
> The adjuster is located between the mixture screws just underneath the center
> fuel inlet. It is caped off from the factory and must be drilled out to gain access to the
> adjuster screw.
Right.
<Good explanation snipped>
I know all that. But, it's just not "easy" to do while going
down the road. That tiny little screw is quite buried behind
and under that inlet fitting (at least on 704xx5x Q-jets).
It would be quite an entertaining visual to watch someone
trying to do that adjustment while going down the road in an
up-to-temp GMC. Not that it couldn't be done, mind you.
>
> Would be nice if it was an allen key head on the screw. I suppose you could probably make one
> for it by grinding a long taper into a long allen headed bolt that was the right thread to fit
> in the slot. Might be a nice adjustment to be able to get to for those that frequently go
> through major altitude changes in their travels.
That's probably true. I'd like to find charts/tables, etc.
that indicate actual effects of this adjustment (i.e.,
fractions of turns=X/1000" of lift or drop=% rich or lean at
Y" of vacuum). Roe is nearly silent, Ruggles is not quite as
silent (pg. 99), Grimsrud and others are basically totally
silent on the issue, except to advise leaving it alone.
Other info I have suggests that performance gain
expectations making this adjustment are misplaced.
Well, one more interesting tidbit of info for the file.
Toby Maki
'73 Glacier 230
Riverside, CA
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| Re: [GMCnet] Q-Jet Replacement [message #90022 is a reply to message #89934] |
Sat, 26 June 2010 18:10 |
g.winger
Messages: 792
Registered: February 2008
Location: Warrenton,Missouri
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On the mains you can not adjust afr for wide open throttle without dissasembly. But thats not what I posted. My last post was rushed,,had to go out of town for a daughters 30th birthday party. So,, you could adjust the mains,,APT of course, with a little help while driving down the road. Might be a little warm and windy of course. APT stands for Adjustable PART Throttle. Now remember and those unfamiler with how it works,(APT).The sping holds it UP and vacume pulls it down. The screw on mine is adjusted with a 5/16 hex bolt with a slot cut in it with a hacksaw, The adjustment is how far DOWN it goes and it adjustes AFR in a high vacume ie. ,,,part throttle. Part throttle= higher vacume, the primary rods are lower in the main jets, witch give you less gas or leaner. More throttle opening,,,lower vacume,, the spring raises the rods exposing more opening of the main jets= richer. Hope this helps clarify what I posted. And I wish I had such a selection of rods and hagers that Tobi has,,I'm just a rookie.PL
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| Re: [GMCnet] Q-Jet Replacement [message #90027 is a reply to message #90022] |
Sat, 26 June 2010 18:48 |
tmaki
Messages: 200
Registered: September 2005
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Paul Leavitt wrote:
>
> On the mains you can not adjust afr for wide open throttle without dissasembly.
Right. I think we had a "Tower of Babel" moment. I
interpreted "mains" as primary, and you used mains again for
secondary. After I figured out what you meant, I knew what
you were saying.
> So,, you could adjust the mains,,APT of course, with a little help while driving down the road.
> Might be a little warm and windy of course.
I'd say so... :-) Deafening and blinding, too.
>APT stands for Adjustable PART Throttle.
Interesting how Rochester changed to a much more accessible
APT in '75, but still plugged the access. One of those "no
user serviceable parts inside" (wink, wink, nudge, nudge)
kinds of thing. They even discouraged their own service
personnel from messing with the APT. Emissions related, I'm
sure.
> And I wish I had such a selection of rods and hagers
I listed my duplicates. If there are any on that list that
you want, let me know. I find all kinds of these things on
my excursions to the Ecology and Pick A Part yards.
Sometimes gold, most times lead.
Toby Maki
'73 Glacier 230
Riverside, CA
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| Re: [GMCnet] Q-Jet Replacement [message #90054 is a reply to message #89934] |
Sat, 26 June 2010 20:21 |
g.winger
Messages: 792
Registered: February 2008
Location: Warrenton,Missouri
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I,m very very lucky, 52,4 kids and 7 grandkids and we might get 9 or 10,,,,,
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