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#1 |
Senior Member
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Location: Cypress, TX
Posts: 3,703
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High idle setting.
This is the tune up sticker for my 1969 CST/10 with 350 engine, Quadrajet, and TH400 automatic transmission. Is there a factory spec for high idle? I don't have the Quadrajet that came with it any more. I don't know if it had a high idle. The current Quadrajet is a model M4ME from 1985-86 and it definitely has a high idle setting.
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#2 |
Registered User
Join Date: Mar 2013
Location: Campbellsville, KY
Posts: 888
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Re: High idle setting.
High idle on my '85 GMC with a 454/Q-jet is 1,900 or 2,000, just as a comparison.
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Alex V. ------ 1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs. 1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue. |
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#3 |
Who Changed This?
![]() Join Date: Jan 2013
Location: Simi Valley, CA
Posts: 10,396
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Re: High idle setting.
You can dial that with the screw. All you need for high idle is that it doesn't die after you start it. If you want to drive right off after you start it, you might have to set the cold high idle higher than if you let it warm up a bit. That also depends on the ambient temperature. Maybe just play with it a bit until it works like you want, since it isn't the original carburetor, already.
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~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA |
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#4 |
Senior Member
![]() Join Date: May 2010
Location: Wisconsin & Arizona
Posts: 4,844
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Re: High idle setting.
The factory service manual for a 350 with Quadrajet specifies 2400 RPM for the fast idle setting.
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#5 |
Senior Member
![]() Join Date: Apr 2016
Location: Cypress, TX
Posts: 3,703
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Re: High idle setting.
Oh, thanks. I completely forgot about the service manual, silly me. I have a PDF of it and want to get a paper copy.
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#6 |
Registered User
Join Date: Jan 2007
Location: Puyallup, WA
Posts: 1,662
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Re: High idle setting.
On my 72, with a 350 and TH350, the sticker on the inner fender says high idle is 1500 and curb idle is 600 in gear. 2400 is waaaaay high
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72 C10 lwb fleetside -stock 350/350 combo |
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#7 | |
Senior Member
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Location: Orem, Utah
Posts: 7,908
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Re: High idle setting.
Quote:
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I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson |
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#8 |
Registered User
Join Date: Apr 2010
Location: So Cal
Posts: 1,437
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Re: High idle setting.
I agree with 68 POS. 2400 is way, way too high. Not only is there no reason for it, cold thick oil puts an unnecessary strain on the distributor drive to the oil pump.
Somewhere around 1200, IMO is usually all that is necessary. With the choke closed, the rich mixture would cause the plugs to foul at normal idle speed of 600. By increasing rpm to a higher idle when cold, the extra air lets the engine run smoother until the engine warms. Choke opening and high idle rpm should be tailored more to your climate conditions than any book setting and only after timing, mixture and idle adjustment are done.
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'67 GMC 2500, 292, 4spd, AC |
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#9 |
Senior Member
![]() Join Date: Apr 2016
Location: Cypress, TX
Posts: 3,703
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Re: High idle setting.
I disconnected the electric wire from the choke to delay the warm up of the choke while I fool with it, and connected my timing light/tachometer. With the engine cold, I pumped the accelerator a couple times and started the engine. I adjusted the high idle to what sounded like a smooth run and nice speed for high idle. I looked at the tach and found it was at 1400.
It appears to have 3 steps of high idle that depend on the temperature. It stays on one step until I lightly press the gas, then it drops down to one of the lower steps or steps off of fast idle completely, depending on how warm the choke is. This is my interpretation of how it is working, I could be mistaken. |
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#10 | |
Who Changed This?
![]() Join Date: Jan 2013
Location: Simi Valley, CA
Posts: 10,396
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Re: High idle setting.
Quote:
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~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA |
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high idle, quadrajet |
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