| Re: [GMCnet] Quadrajet problems [message #138217 is a reply to message #137646] |
Tue, 09 August 2011 21:31   |
Brian Waddell
Messages: 409
Registered: March 2010
Karma: -4
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Senior Member |
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electronic ignition module....brian 77 ele
> To: gmclist@temp.gmcnet.org
> From: kwharland@bellsouth.net
> Date: Fri, 5 Aug 2011 17:59:17 -0500
> Subject: [GMCnet] Quadrajet problems
>
>
>
> Unfortunately we left our GMC sit pretty much unused for several years. I did manage to start it periodically and let it get up to operating temps, same with the genset and roof A/C's.
>
> This time I went to take it out for a drive and found that it stalls whenever I touch the throttle. So I pulled the carb, cleaned it and installed new gaskets, accelerator pump and inlet filter. It still bogs and tries to stall whenever I press on the pedal. I'm an adept mechanic but I'm stumped here. I've had that carb off and on a dozen times, cleaned it, blew it out, tweaked it - no change. I know I can send it off but again, I've generally competent and unwilling to simply give up. A good friend and shop owner is out of ideas as well.
>
> The transition from idle to part open is smooth, accel pumps works well, but just beyond barely open it appears to starve for gas. Float level has been checked and rechecked. Meter rod piston rises as expected. Closed off all vac ports with no change.
>
> Ideas?
> --
> 1978 Eleganza II
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| Re: [GMCnet] Quadrajet problems [message #138554 is a reply to message #138217] |
Thu, 11 August 2011 19:22 |
kwharland
Messages: 246
Registered: November 2005
Location: Central Florida
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Problem solved!
The problem was twofold, part carburetor, part ignition timing. In retrospect, the carb part is funny. It reminds me of a breakdown I suffered with my hotrod Ford back in the sixties. It had a dual point ignition and it start running poorly on a trip and I recognized the problem as ignition related. I'd pop the hood, pull off the cap and rotor and study the points. I'd bump the starter and watch the points open and close and all appeared well. Put the rotor and cap back on, still backfiring and running ragged. Off with the cap and rotor, check gap on points, try again. Same result! Into it again and swap out condenser. Back together and still misfiring. One more time and in the process of putting the rotor and cap on...and trust me, I looked at the inside of that cap every time, I noticed the arm on the rotor was cocked! It apparently hit one of the contacts in the cap and broke loose and moved. New rotor and ran perfectly.
I know, everyone's asking what's this got to do with your GMC! Well, during the last dis-assembly of the carb, I happen to look at the underside of the air horn. Low and behold, in the cavity where the metering rods rise up, there was a compressed piece of gasket! Remember, the original gasket was in poor shape and I replaced it the first time I took it apart. Apparently the PO or his mechanic did not insure that the piston and metering rods passed through the cutouts in the gasket when they installed the top of the carb and that portion of the gasket jammed up inside.
That was limiting the travel of the rods and keeping the mixture too lean.
The other portion of the problem was that the vacuum advance was connected to the thermostatically controlled vacuum valve which is not opening. Thus no advance! Moved it over to a port connection and it runs fine.
Thanks to everyone for their suggestions!
1978 Eleganza II
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| Re: [GMCnet] Quadrajet problems [message #138571 is a reply to message #138554] |
Thu, 11 August 2011 19:51 |
C Boyd
Messages: 2629
Registered: April 2006
Karma: 18
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The other portion of the problem was that the vacuum advance was connected to the thermostatically controlled vacuum valve which is not opening. Thus no advance! Moved it over to a port connection and it runs fine.
Thanks to everyone for their suggestions![/quote]
Sir: the olds dist is designed to work on ported vacuum (no vaccum at idle). The thermostatic valve allows switching to manifold vacuum to the dist only when the engine temp gets around 205. This allows the engine to idle faster as in traffic on a hot day with ac on. Most people do bypass it, but hook it to the ported vacuum port on the carb and not the manifold vacuum port. Using manifold vacuum can contribute to dieseling when shut off.
C. Boyd
76 Crestmont
East Tennessee
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| Re: [GMCnet] Quadrajet problems [message #138615 is a reply to message #138571] |
Thu, 11 August 2011 23:02 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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| C Boyd wrote on Thu, 11 August 2011 19:51 |
The other portion of the problem was that the vacuum advance was connected to the thermostatically controlled vacuum valve which is not opening. Thus no advance! Moved it over to a port connection and it runs fine.
Thanks to everyone for their suggestions!
|
Sir: the olds dist is designed to work on ported vacuum (no vacuum at idle). The thermostatic valve allows switching to manifold vacuum to the dist only when the engine temp gets around 205. This allows the engine to idle faster as in traffic on a hot day with ac on. Most people do bypass it, but hook it to the ported vacuum port on the carb and not the manifold vacuum port. Using manifold vacuum can contribute to dieseling when shut off.[/quote]
I was going to post the same thing but you beat me to it. I found my old TVS in the basement a few weeks ago. I removed it many years ago. If the TVS is bypassed connect the vacuum advance hose to to "ported vacuum" port vacuum carb.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Quadrajet problems [message #138621 is a reply to message #137646] |
Thu, 11 August 2011 23:27 |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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| kwharland wrote on Fri, 05 August 2011 15:59 |
The transition from idle to part open is smooth, accel pumps works well, but just beyond barely open it appears to starve for gas. Float level has been checked and rechecked. Meter rod piston rises as expected. Closed off all vac ports with no change.
Ideas?
|
What kind of distributor? Spark can do this, too. As you open the throttle, more air is let in and in essence, the compression rises. Spark doesn't jump as well in dense high pressure as it does in high vacuum low pressure situations.
Checking the spark in open air is not good enough. Check to see if you really have a hot, blue snap of a spark. Look at the rotor and weights. The old rotors will crack on the underside along the ridge and let the spark arc onto the advance weights. Had an engine that would die if you even touched the gas but would start and idle fine.
The coil can be suspect with low spark issues.
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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| Re: [GMCnet] Quadrajet problems [message #138660 is a reply to message #138554] |
Fri, 12 August 2011 07:43 |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
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Great job Ken !! Thanks for sharing the experience, we need all the info we
can get on things like this.
Ray
Ray & Lisa Erspamer
78 Royale "Great Lakes Eagle"
Center Kitchen TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMC-Royale@att.net
414-745-3188
Web Site: http://ray-lisa.page.tl/
----- Original Message ----
From: Ken Harland <kwharland@bellsouth.net>
To: gmclist@temp.gmcnet.org
Sent: Thu, August 11, 2011 7:22:53 PM
Subject: Re: [GMCnet] Quadrajet problems
Problem solved!
The problem was twofold, part carburetor, part ignition timing. In retrospect,
the carb part is funny. It reminds me of a breakdown I suffered with my hotrod
Ford back in the sixties. It had a dual point ignition and it start running
poorly on a trip and I recognized the problem as ignition related. I'd pop the
hood, pull off the cap and rotor and study the points. I'd bump the starter and
watch the points open and close and all appeared well. Put the rotor and cap
back on, still backfiring and running ragged. Off with the cap and rotor, check
gap on points, try again. Same result! Into it again and swap out condenser.
Back together and still misfiring. One more time and in the process of putting
the rotor and cap on...and trust me, I looked at the inside of that cap every
time, I noticed the arm on the rotor was cocked! It apparently hit one of the
contacts in the cap and broke loose and moved. New rotor and ran perfectly.
I know, everyone's asking what's this got to do with your GMC! Well, during the
last dis-assembly of the carb, I happen to look at the underside of the air
horn. Low and behold, in the cavity where the metering rods rise up, there was
a compressed piece of gasket! Remember, the original gasket was in poor shape
and I replaced it the first time I took it apart. Apparently the PO or his
mechanic did not insure that the piston and metering rods passed through the
cutouts in the gasket when they installed the top of the carb and that portion
of the gasket jammed up inside.
That was limiting the travel of the rods and keeping the mixture too lean.
The other portion of the problem was that the vacuum advance was connected to
the thermostatically controlled vacuum valve which is not opening. Thus no
advance! Moved it over to a port connection and it runs fine.
Thanks to everyone for their suggestions!
--
1978 Eleganza II
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Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
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| Re: [GMCnet] Quadrajet problems [message #138733 is a reply to message #138661] |
Fri, 12 August 2011 15:21 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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| CBWoodSR wrote on Fri, 12 August 2011 07:48 |
Picture of the port???
Thankx
|
It is the highest port on the front (driver's side) of the carb. (Unless you are in Australia.)
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Quadrajet problems [message #138775 is a reply to message #138733] |
Fri, 12 August 2011 21:27 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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Yep,
Here it's the lowest port! ;-)
Regards,
Rob "Upside Down" M.
Sydney, Australia
-----Original Message-----
From: Ken Burton
It is the highest port on the front (driver's side) of the carb. (Unless
you are in Australia.)
--
Ken
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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