Home » Public Forums » GMCnet » [GMCnet] Q Jet Set up
| [GMCnet] Q Jet Set up [message #142436] |
Mon, 05 September 2011 09:33  |
Arthur Mansfield
Messages: 290
Registered: April 2010
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I am trying to adjust my Carb. I do not have a stock cam. My vacuum at idle seems lower than normal. I was wondering what is the normal level of vacuum at idle. My reading run 13 to14 on the vacuum gage. I am thinking I need to change the spring on the power piston.
Art & Doris
76 EL
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| Re: [GMCnet] Q Jet Set up [message #142453 is a reply to message #142436] |
Mon, 05 September 2011 12:14  |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
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| Arthur Mansfield wrote on Mon, 05 September 2011 07:33 |
I am trying to adjust my Carb. I do not have a stock cam. My vacuum at idle seems lower than normal. I was wondering what is the normal level of vacuum at idle. My reading run 13 to14 on the vacuum gage. I am thinking I need to change the spring on the power piston.
Art & Doris
76 EL
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Art,
This is pretty long, and I am sorry about that. If you don't want to read all below, just be reeeeal careful fiddling with the Power Valve.
I no longer have a carburetor, but have experimented with the power valve concept (called Power Enrichment in fuel injection talk). Power valve or PE are really misnomers in a way, because while the enrichment does give a bit more power its main purpose is to cool the combustion. I know using gasoline is an expensive way to cool combustion but there you have it.
With the adjustable setting on my Fuel Injection, I raised the throttle postion that would cause it to jump into PE. I also was fiddling with Lean Cruise. Many doomsdayers were telling me Lean Cruise would burn valves. I got exhaust temperature gauges. I learned quite a bit in a hurry.
1) Lean Cruise runs cooler the than normal 14.7 air/fuel ratio.
2) Staying in regular 14.7 and not going to Power (valve) Enrichment cause heat to go up... and really fast. I am talking 150 degrees in 30 seconds @ 2500 RPMS. I have talked to guys who have burned valves at 1350 and right now with wife driving we are running on flat Nebraska land at a fuzz over 1100.
Needless to say, my Power settings are back to GM's numbers.
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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| Re: [GMCnet] Q Jet Set up [message #142454 is a reply to message #142453] |
Mon, 05 September 2011 12:19 |
fred v
Messages: 999
Registered: April 2006
Location: pensacola, fl.
Karma: 0
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at idle i'm at about 18"; cruise on flat/level i run about 14. power valve come on at about 9 or is it 6? bad memory.
Fred V
'77 Royale RB 455
P'cola, Fl
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| Re: [GMCnet] Q Jet Set up [message #142461 is a reply to message #142453] |
Mon, 05 September 2011 12:42 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Kinda reminds me of the story that I heard once about the farmer who had a
plow horse, and he figured that if he cut back on the horse's daily ration
of oats that he could save money, and if he did it a little bit at a time
that the horse wouldn't even miss it. Just about the time he had the horse
weaned off the oats, the horse up and died. Kinda like leaning out the high
speed circuits in either fuel injection or carbs. About the time you see
some real results in fuel economy, the engine will up and die. That is a
VERY FINE line between fragging the engine and making the most horsepower. I
have learned that lesson the very hard way more than once. When you are on
the ragged edge of maximum fuel economy, even slight changes in atmospherics
or altitude can spell doom. Keep it on the rich side on high speed/high
power settings. Gasoline is waaayy cheaper than pistons and valves.
Jim Hupy
Salem, OR
78 GMC Royale 403
On Mon, Sep 5, 2011 at 10:14 AM, George Beckman <gbeckman@pggp.com> wrote:
>
>
> Arthur Mansfield wrote on Mon, 05 September 2011 07:33
> > I am trying to adjust my Carb. I do not have a stock cam. My vacuum at
> idle seems lower than normal. I was wondering what is the normal level of
> vacuum at idle. My reading run 13 to14 on the vacuum gage. I am thinking I
> need to change the spring on the power piston.
> >
> > Art & Doris
> > 76 EL
>
> Art,
>
> This is pretty long, and I am sorry about that. If you don't want to read
> all below, just be reeeeal careful fiddling with the Power Valve.
>
> I no longer have a carburetor, but have experimented with the power valve
> concept (called Power Enrichment in fuel injection talk). Power valve or PE
> are really misnomers in a way, because while the enrichment does give a bit
> more power its main purpose is to cool the combustion. I know using
> gasoline is an expensive way to cool combustion but there you have it.
>
> With the adjustable setting on my Fuel Injection, I raised the throttle
> postion that would cause it to jump into PE. I also was fiddling with Lean
> Cruise. Many doomsdayers were telling me Lean Cruise would burn valves. I
> got exhaust temperature gauges. I learned quite a bit in a hurry.
>
> 1) Lean Cruise runs cooler the than normal 14.7 air/fuel ratio.
>
> 2) Staying in regular 14.7 and not going to Power (valve) Enrichment cause
> heat to go up... and really fast. I am talking 150 degrees in 30 seconds @
> 2500 RPMS. I have talked to guys who have burned valves at 1350 and right
> now with wife driving we are running on flat Nebraska land at a fuzz over
> 1100.
>
> Needless to say, my Power settings are back to GM's numbers.
> --
> '74 Eleganza, SE, Howell + EBL
> Best Wishes,
> George
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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| Re: [GMCnet] Q Jet Set up [message #142466 is a reply to message #142436] |
Mon, 05 September 2011 13:44 |
roy1
Messages: 2126
Registered: July 2004
Location: Minden nevada
Karma: 6
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If you at a higher elevation say 4,000 ft the vacuum will be lower. I have seen vacuum gauges that were not accurate did you check it against a known good gage? Changing the power valve won't have any effect on what vacuum the engine idles.
Roy
Roy Keen
Minden,NV
76 X Glenbrook
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| Re: [GMCnet] Q Jet Set up [message #142493 is a reply to message #142461] |
Mon, 05 September 2011 17:29 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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There was a guy named Don Vesco who was famous for getting obscene amounts of power out of ringding motorcycle engines in races. He told somebody "If it burns through the pistons on the cool-down lap, we got it right." My mileage trade says the engine dies about the same time the transmission dies and the wheels go square.
--johnny
--- On Mon, 9/5/11, James Hupy <jamesh1296@gmail.com> wrote:
From: James Hupy <jamesh1296@gmail.com>
Subject: Re: [GMCnet] Q Jet Set up
To: gmclist@temp.gmcnet.org
Date: Monday, September 5, 2011, 5:42 PM
Kinda reminds me of the story that I heard once about the farmer who had a
plow horse, and he figured that if he cut back on the horse's daily ration
of oats that he could save money, and if he did it a little bit at a time
that the horse wouldn't even miss it. Just about the time he had the horse
weaned off the oats, the horse up and died. Kinda like leaning out the high
speed circuits in either fuel injection or carbs. About the time you see
some real results in fuel economy, the engine will up and die. That is a
VERY FINE line between fragging the engine and making the most horsepower. I
have learned that lesson the very hard way more than once. When you are on
the ragged edge of maximum fuel economy, even slight changes in atmospherics
or altitude can spell doom. Keep it on the rich side on high speed/high
power settings. Gasoline is waaayy cheaper than pistons and valves.
Jim Hupy
Salem, OR
78 GMC Royale 403
On Mon, Sep 5, 2011 at 10:14 AM, George Beckman <gbeckman@pggp.com> wrote:
>
>
> Arthur Mansfield wrote on Mon, 05 September 2011 07:33
> > I am trying to adjust my Carb. I do not have a stock cam. My vacuum at
> idle seems lower than normal. I was wondering what is the normal level of
> vacuum at idle. My reading run 13 to14 on the vacuum gage. I am thinking I
> need to change the spring on the power piston.
> >
> > Art & Doris
> > 76 EL
>
> Art,
>
> This is pretty long, and I am sorry about that. If you don't want to read
> all below, just be reeeeal careful fiddling with the Power Valve.
>
> I no longer have a carburetor, but have experimented with the power valve
> concept (called Power Enrichment in fuel injection talk). Power valve or PE
> are really misnomers in a way, because while the enrichment does give a bit
> more power its main purpose is to cool the combustion. I know using
> gasoline is an expensive way to cool combustion but there you have it.
>
> With the adjustable setting on my Fuel Injection, I raised the throttle
> postion that would cause it to jump into PE. I also was fiddling with Lean
> Cruise. Many doomsdayers were telling me Lean Cruise would burn valves. I
> got exhaust temperature gauges. I learned quite a bit in a hurry.
>
> 1) Lean Cruise runs cooler the than normal 14.7 air/fuel ratio.
>
> 2) Staying in regular 14.7 and not going to Power (valve) Enrichment cause
> heat to go up... and really fast. I am talking 150 degrees in 30 seconds @
> 2500 RPMS. I have talked to guys who have burned valves at 1350 and right
> now with wife driving we are running on flat Nebraska land at a fuzz over
> 1100.
>
> Needless to say, my Power settings are back to GM's numbers.
> --
> '74 Eleganza, SE, Howell + EBL
> Best Wishes,
> George
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] Q Jet Set up [message #142494 is a reply to message #142436] |
Mon, 05 September 2011 17:31 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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As far as the next larger rod.... Primary or secondary?
the primaries are by the number stamped on them. The secondaries had dual letter codes you need a chart to crack. Larger primary rods will make the normal mixture leaner and the power mixture the same as the tips are the same on the primaries. Not so on the secodaries where the base and tip have sizes specific to the code. Be carefull though the GMC used a unique primary rod with a specific dual taper and I think this is the only Qjet that used it. If I can find my Qjet book I'll scan the appropriate chart pages for primary rods, jets, secondary rods and power spring colors.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: [GMCnet] Q Jet Set up [message #142533 is a reply to message #142491] |
Mon, 05 September 2011 21:52 |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
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Senior Member |
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Art,
Look here ...
http://www.gmcws.org/Tech/quadrajet/index_files/frame.htm
and Brent Covey's comments here
http://gmcmotorhome.info/engine.html#carb
Dennis
| Arthur Mansfield wrote on Mon, 05 September 2011 17:18 |
Thanks Everyone
I live in AL so I do not have an altitude problem. My exhaust has always smelled rich. My vacuum gage reads 13 to 14 when idling. I do not remember what the vacuum ran yesterday when I test drove it after reseting the timing. I went back to the book setting of 8 degree. It was a off about two degree when set using Jim B's method of vacuum and how it starts. My engine only has about 15K since it was rebuilt so the timing chain should be in good shape.
My engine has always run hotter than I think it should. I installed an AL radiator and it helped but the clutch fan has to run about 25% (comes on at about 195 & goes off at 185) of the time when on level ground at 65mph at 95 degree out side. My gas milage has been ok ... around 9 mpg at 60 to 65 mph. I have always thought I was low on power when I got on it. I am going to change the primary jets to 73. I want 72 but could not get them locally. Does anyone know what the next size larger rods number is. As one may guess I like to tinker and usually end up back to where I started. I have been going to the web looking for articles as to how to set up the Q Jet. The one I am using is How to Tune a Q-Jet (basic) by Lars Grimsrud. Most articles are how to set up for drag racing and not normal driving.
Art & Doris
EL 76
On Sep 5, 2011, at 1:44 PM, roy@gmcnet.org, keen@gmcnet.org, Minden@gmcnet.org, Nv '76 Glenbrook wrote:
>
>
> If you at a higher elevation say 4,000 ft the vacuum will be lower. I have seen vacuum gauges that were not accurate did you check it against a known good gage? Changing the power valve won't have any effect on what vacuum the engine idles.
> Roy
> _______________________________________________
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Dennis S
73 Painted Desert 230
Memphis TN Metro
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| Re: [GMCnet] Q Jet Set up [message #142551 is a reply to message #142499] |
Tue, 06 September 2011 00:10 |
 |
mike miller
Messages: 3576
Registered: February 2004
Location: Hillsboro, Oregon
Karma: 0
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| Arthur Mansfield wrote on Mon, 05 September 2011 15:50 |
John
I have larger primary jets and may want to put different rods in if the WOT works good and the cruise is to rich. I have a list of secondary rods but have not found a list of the primary jets. ...
|
I am assuming you have an O2 sensor installed. It would be very hard to tweak a Q-jet in without one.
Inventory your Q-jet and ensure it has the correct (and un-modified) parts. You never know what has happened to it over the years.
If you have a REAL un-modified motorhome Q-jet, in good condition, I would not mess with it. GM engineers spent a long time tweaking and making special parts, to make it work in our application. They actually did a pretty good job. (You would need to go to a modern fuel injection system to do better.)
If it doesn't work correctly, maybe you have a problem elsewhere?
OBTW: Years ago, I bought a bag of the correct (red) power valve springs.
<http://www.gmcmhphotos.com/photos/showphoto.php?photo=8451>
I THINK I still have a few left... I used one, sold some at a rally, mailed two... (never did get paid  ) should be two left around here... somewhere.
Mike Miller -- Hillsboro, OR -- on the Black list
(#2)`78 23' Birchaven Rear Bath -- (#3)`77 23' Birchaven Side Bath
More Sidekicks than GMC's and a late model Malibu called 'Boo'
http://m000035.blogspot.com
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| Re: [GMCnet] Q Jet Set up [message #142552 is a reply to message #142491] |
Tue, 06 September 2011 00:09 |
Guy Lopes
Messages: 499
Registered: April 2004
Location: Sacramento, CA
Karma: 3
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Senior Member |
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Art,
I went looking for that pdf by Lars. In my Google search I found several
versions of it. I decided to email Lars and ask about the different versions
and where I could find the most recent revision. I promptly emailed me back
with the most current versions of the following files:
How to Tune a Q-Jet (basic) - Rev. BD 5-1-11
Problems I have Seen with Commercially Rebuilt Q-Jets - Rev D 10-2-10
Q-Jet Problems I have Seen - Rev H 1-2-10
These are very well written and comprehensive.
I asked if I could post these and Lars responded that since they are
copyrighted and undergo frequent updates and revisions, he doesn't want them
posted on web sites and forums. So, if you would like copies of your own,
simply email Lars at: v8fastcars@msn.com I just told him a little about my
involvement with the classic GMC and requested the files.
Guy in Sacramento
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org
[mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of 1104agm
Sent: Monday, September 05, 2011 3:19 PM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] Q Jet Set up
Thanks Everyone
<snip>
for articles as to how to set up the Q Jet. The one I am using is How to
Tune a Q-Jet (basic) by Lars Grimsrud. Most articles are how to set up for
drag racing and not normal driving.
Art & Doris
EL 76
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Guy Lopes
76 Birchaven "Orion"
Sacramento, CA
W6TOL
www.GMC-Guy.com
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| Re: [GMCnet] Q Jet Set up [message #142581 is a reply to message #142551] |
Tue, 06 September 2011 10:17 |
Arthur Mansfield
Messages: 290
Registered: April 2010
Karma: 0
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Senior Member |
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Mike
I agree that if I had not changed the cam, the carb would be very close if not ideal for our set up. My vacuum is way off at idle. The vacuum reading runs between 13 & 14 at 650 RPM.
My RV has always run hotter than it should even with an AL radiator. I played with the timing and set it like Jim B showed me and set it by the book. Neither helped and they were almost the same.
So I decided to start changing the carb. I do not have a oxygen sensor and don't even know what to buy to use to read an oxygen senor to set the carbs with.
I am going to go by performance (acceleration, engine temperature & looking at the plugs) to set the carb. My first change was going to be to change the jets and then change the spring on the power valve. I order a set of springs for the carb. If the springs come in first I will change the spring before I change the jets.
Art & Doris
76 EL
Decatur, AL
On Sep 6, 2011, at 12:10 AM, Mike Miller wrote:
>
>
> Arthur Mansfield wrote on Mon, 05 September 2011 15:50
>> John
>>
>> I have larger primary jets and may want to put different rods in if the WOT works good and the cruise is to rich. I have a list of secondary rods but have not found a list of the primary jets. ...
>
>
> I am assuming you have an O2 sensor installed. It would be very hard to tweak a Q-jet in without one.
>
> Inventory your Q-jet and ensure it has the correct (and un-modified) parts. You never know what has happened to it over the years.
>
> If you have a REAL un-modified motorhome Q-jet, in good condition, I would not mess with it. GM engineers spent a long time tweaking and making special parts, to make it work in our application. They actually did a pretty good job. (You would need to go to a modern fuel injection system to do better.)
>
> If it doesn't work correctly, maybe you have a problem elsewhere?
>
> OBTW: Years ago, I bought a bag of the correct (red) power valve springs.
> <http://www.gmcmhphotos.com/photos/showphoto.php?photo=8451>
>
> I THINK I still have a few left... I used one, sold some at a rally, mailed two... (never did get paid :cry: ) should be two left around here... somewhere.
> --
> Mike Miller -- Hillsboro, OR -- on the Black list
> (#1)'73 26' exPainted D. -- (#2)`78 23' Birchaven Rear Bath -- (#3)`77 23' Birchaven Side Bath
> http://m000035.blogspot.com
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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| Re: [GMCnet] Q Jet Set up [message #142596 is a reply to message #142581] |
Tue, 06 September 2011 11:59 |
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mike miller
Messages: 3576
Registered: February 2004
Location: Hillsboro, Oregon
Karma: 0
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| Arthur Mansfield wrote on Tue, 06 September 2011 08:17 |
... I agree that if I had not changed the cam, the carb would be very close if not ideal for our set up. ...
... So I decided to start changing the carb. I do not have a oxygen sensor and don't even know what to buy to use to read an oxygen senor to set the carbs with. ...
|
1. It is your coach and you will need to 'live with' your changes... you can do what you want.
2. Rather than tweaking a 'old tech' carb, I highly recommend a fuel injection system using a "Dynamic EFI" "EBL" system.
<http://www.dynamicefi.com/>
With it you'll have a fuel injection system based on GM parts, that has a port to connect a computer and adjust the "carb" settings pretty much on the fly. PLUS you'll have REAL numbers on what these changes have done for you. There is a good web site where the GMC'ers who are into this modification discuss and help each other... most of them also read and post here.
<http://groups.google.com/group/gmcmh-efi?hl=en>
3. An O2 sensor and a simple meter will provide "REAL numbers" on what your changes to the carb have done for you. You would just have to learn to interpret what you see. (Do a Google search on "reading o2 sensor carburetor" for LOTS of info.)
NOTE: With the newer tech involved you do not need to be a 'computer nerd' to connect a fairly standard low end laptop to a GMC with fuel injection. Most of it is just buying the correct parts.
Mike Miller -- Hillsboro, OR -- on the Black list
(#2)`78 23' Birchaven Rear Bath -- (#3)`77 23' Birchaven Side Bath
More Sidekicks than GMC's and a late model Malibu called 'Boo'
http://m000035.blogspot.com
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| Re: [GMCnet] Q Jet Set up [message #142743 is a reply to message #142436] |
Wed, 07 September 2011 21:51 |
kingd
Messages: 592
Registered: June 2004
Karma: 2
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Senior Member |
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Art, you say you have a different cam, but is it a performance cam for a car or does it actually have the grind for the reequirements of an RV. "BIG" cams, eg for high performance cars, have low idle vacuum. If I were you I'd try to find the real specs for the cam you have and compare to a stock cam. I would suspect Jim Bounds wouldn't want to reveal the numbers for his special cam. (why would he, it's his baby) Openining point, closing point and max lift are just part of a cam. You may want to compare your cam to a stock cam or to one someone has that works well at every 10 degrees of crank rotation. If your cam is not suitable for an RV, drastic action might be required. Power valves and main jets should do NOTHING for vacuum at idle.
DAVE KING (who has had to check racing cams at 5 degrees of cam rotation)
DAVE KING
lurker, wannabe
Toronto, Ontario, Canada
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