| Idle increase with A/C? [message #333487] |
Thu, 14 June 2018 09:52  |
mballar
Messages: 20
Registered: March 2018
Location: West Central Missouri
Karma: 0
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On the 1975 455, was there a mechanism to bump up the idle speed when the A/C was turned on? My carb doesn't seem to have anything like that and turning on the A/C will often kill the engine.
Mike & Apryl Ballard
Fair Play, Missouri
1975 Glenbrook
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| Re: Idle increase with A/C? [message #333489 is a reply to message #333487] |
Thu, 14 June 2018 11:14  |
bwevers
Messages: 597
Registered: October 2010
Location: San Jose
Karma: 5
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What's your idle speed?
Maybe it's too low.
Bill Wevers GMC49ers, GMC Western States
1975 Glenbrook - Manny Powerdrive, OneTon
455 F Block, G heads
San Jose
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| Re: Idle increase with A/C? [message #333490 is a reply to message #333487] |
Thu, 14 June 2018 11:27 |
Rick Staples
Messages: 126
Registered: May 2014
Location: Johnstown, Colorado, USA
Karma: -1
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I agree that if it kills the engine when the A/C comes on, the idle may be too low, or otherwise misadjusted. However, I also agree that the GMC SHOULD have some sort of "idle up" system. My early '75 455 doesn't have one either, and idling in traffic or waiting in a parking lot on one of our brutal Colorado summer days, the A/C (2 evaporator system) loses much of its cooling power. I'm thinking about switching the vacuum advance to full manifold vacuum when the A/C compressor kicks in, if I can find an appropriate solenoid vacuum switching valve. A throttle kicker is another possibility. Maybe both! (I hate sweating in the cab!)
Good luck, and stay cool.
Rick Staples
Rick Staples, '75 Eleganza, Johnstown, CO
"Advice is a dangerous gift, even from the Wise to the Wise, and all paths may run ill." -Tolkien
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| Re: Idle increase with A/C? [message #333497 is a reply to message #333487] |
Thu, 14 June 2018 17:47 |
Bullitthead
Messages: 1411
Registered: November 2013
Karma: 5
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I think the Cadillacs of the early "70s were the pioneers of using the idle up solenoid on the throttle. Might require that particular carb base though.
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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| Re: Idle increase with A/C? [message #333498 is a reply to message #333487] |
Thu, 14 June 2018 18:41 |
bobby5832708
Messages: 237
Registered: November 2006
Location: Winter Springs FL
Karma: 3
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The original carb on my early 1974 GMC did not have an idle up solenoid.
I made the following mod about 10 years ago. My memory sucks so the specifics may be a bit off but the mod still works fine to this day.
So what I did was use (I think) an egr solenoid from a GM 4.3 and blocked off the vent port. This left me with a 2 port air valve, flow when power is applied, blocked when power is off. Wired the coil to be energized when the ac compressor is on. Vacuum lines, one from manifold to supply vacuum and the other teed into the line going to the vacuum advance. On the ported vacuum port in the carb base, I filled it with jb weld and drilled it to something small, like maybe a number 58 or so drill bit.
What this does is at idle with no ac the timing is not vacuum advanced. RPM's are 580 to 600 in drive. Off idle and the port is exposed to vacuum and the vacuum can operates like it always did and the timing vacuum advances. At idle if the ac is turned on the electric valve opens and applies full vacuum to the vacuum can and the timing vacuum advances. Idle RPM's go to 620 to 640 in drive. The slight vacuum leak from the ported vacuum port leans the mixture about .1 (point one) on the fuel ratio gauge. Off idle the ported vacuum port gets vacuum and, in addition to the vacuum coming thru the electric valve, the vacuum can works like you would expect from a full-time vacuum setup (which it is). If the ac cycles on and off at idle I can barely notice the difference in engine speed, listening very carefully I can hear the change in the exhaust note.
I'm using a Crane adjustable vacuum can so everything is tuneable. I couldn't tell you how much advance or at what vacuum it starts at, I didn't write that down. All I know is that it still works fine after all these years and wonder why GM didn't do something like this originally (money?).
JWID
Bob Heller
2017 Winnebago 29VE
Winter Springs FL
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| Re: [GMCnet] Idle increase with A/C? [message #333508 is a reply to message #333498] |
Fri, 15 June 2018 00:03 |
jimk
Messages: 6734
Registered: July 2006
Location: Belmont, CA
Karma: 9
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Senior Member |
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During the warmer season, one should set the idel speed with the a/c
compressor running and in drive and on a slightly higher rpm.
Being a larger displacement engine, we find it does the job.
Also you should NEVER remove the Thermostatic Vaccume Switch as if you get
in a traffic and engine with A/C on a warm day the engine will over heat
even with a HD fan clutch. TheTVS will pull full vaccume on the distributor
and increase the rpm to 1,500, which is enough to allow tthe fan to to cool
the engine.
On Thu, Jun 14, 2018 at 4:41 PM, Bob Heller wrote:
> The original carb on my early 1974 GMC did not have an idle up solenoid.
>
> I made the following mod about 10 years ago. My memory sucks so the
> specifics may be a bit off but the mod still works fine to this day.
>
> So what I did was use (I think) an egr solenoid from a GM 4.3 and blocked
> off the vent port. This left me with a 2 port air valve, flow when power is
> applied, blocked when power is off. Wired the coil to be energized when
> the ac compressor is on. Vacuum lines, one from manifold to supply vacuum
> and
> the other teed into the line going to the vacuum advance. On the ported
> vacuum port in the carb base, I filled it with jb weld and drilled it to
> something small, like maybe a number 58 or so drill bit.
>
> What this does is at idle with no ac the timing is not vacuum advanced.
> RPM's are 580 to 600 in drive. Off idle and the port is exposed to vacuum
> and
> the vacuum can operates like it always did and the timing vacuum advances.
> At idle if the ac is turned on the electric valve opens and applies full
> vacuum to the vacuum can and the timing vacuum advances. Idle RPM's go to
> 620 to 640 in drive. The slight vacuum leak from the ported vacuum port
> leans the mixture about .1 (point one) on the fuel ratio gauge. Off idle
> the ported vacuum port gets vacuum and, in addition to the vacuum coming
> thru
> the electric valve, the vacuum can works like you would expect from a
> full-time vacuum setup (which it is). If the ac cycles on and off at idle I
> can
> barely notice the difference in engine speed, listening very carefully I
> can hear the change in the exhaust note.
>
> I'm using a Crane adjustable vacuum can so everything is tuneable. I
> couldn't tell you how much advance or at what vacuum it starts at, I didn't
> write
> that down. All I know is that it still works fine after all these years
> and wonder why GM didn't do something like this originally (money?).
>
> JWID
>
> --
> Bob Heller
> 1974 X-Canyonlands 26ft
> Original 455 exc for timing chain,
> Rockwell intake, valve covers. 141k miles.
> Winter Springs FL
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
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>
--
Jim Kanomata
Applied/GMC, Newark,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: Idle increase with A/C? [message #333514 is a reply to message #333487] |
Fri, 15 June 2018 07:53 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Senior Member |
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If I attempt to idle my coach 1500 RPM in gear with the brake on, it will quickly overheat. Hate to think what it's doing to the transmission. I don't think I'd set it up that far. All the carbureted air conditioned stuff I had in the late 60s and 70s had a solenoid connected to the compressor clutch which bumped the throttle a bit.
--johnny
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: Idle increase with A/C? [message #333532 is a reply to message #333497] |
Fri, 15 June 2018 11:17 |
mballar
Messages: 20
Registered: March 2018
Location: West Central Missouri
Karma: 0
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Junior Member |
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I see idle solenoids available for 1975 Quadrajets that might work. Worth a shot anyway.
Mike & Apryl Ballard
Fair Play, Missouri
1975 Glenbrook
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| Re: Idle increase with A/C? [message #333538 is a reply to message #333487] |
Fri, 15 June 2018 12:07 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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GM did exactly this on some 70s cars. I have a 76 Grand LeMans with Pontiac 350/2. There is an idle stop solenoid that increases idle speed about 50 RPM when AC is requested. With the added A6 load the result is no RPM LOSS. Keep in mind that 76 was the last year before CCOT compressor cycling became the new standard. The solenoid needs enough oomph to bump the throttle each cycle start. When the ECMs started, AC request was through the computer and the AC clutch relay. The IAC (or carb solenoid) was given a signal to increase RPM and the clutch was delayed .4 second to allow engine RPM to rise as clutch was engaging
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Idle increase with A/C? [message #333540 is a reply to message #333487] |
Fri, 15 June 2018 12:11 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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However this was used more on "anemic" 4 and 6 cyl models. With the 455 you should be able to set a good idle speed and be fine. I prefer the higher end of the spec range for better cooling and oil throw from the crank up to pistons and walls to lube and cool.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Idle increase with A/C? [message #333545 is a reply to message #333487] |
Fri, 15 June 2018 13:08 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Senior Member |
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I think you're right. Our old Buick didn't have one. some of the others did. Set the idle correctly with the A/C off, then engage the compressor and solenoid, and adjust the plunger bolt for the same idle as before. The dealer installation in a bud's Dad's '58 Chev had a pull knob to increase the idle. Most were misadjusted to the point you could use them as a no - cancel cruise control.
--johnny
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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