Idle variance due to temperature?
Thread Starter
Racer
Joined: Oct 2005
Posts: 449
Likes: 0
From: Boulder CO
Idle variance due to temperature?
Have a '72 sb, 4sp with Q-Jet. This winter, I set the idle (along with mixture) to 750rpm and the few times I've driven the car in cool weather, all has been well and the idle has remained constant. Today temp is in the mid-70s and I have been driving on the highway at 70mph. Coming to a stop, the idle drops down to 500rpm and remains there or a little less, running rough. When hot starting, I need to keep a little more throttle on to prevent her from stalling. Question is, should the higher temperature affect the idle? Or as I suspect, is there another adjustment that I'm missing that would cause this symptom?
Thread Starter
Racer
Joined: Oct 2005
Posts: 449
Likes: 0
From: Boulder CO
Originally Posted by icanfixitforyou
Time to adjust the mixture screws for the warmer weather.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
No, the outside air temp won't affect Q-Jet idle.
Why don't you bring the car over so we can take a look at it? Sounds like a basic tuning issue.
Why don't you bring the car over so we can take a look at it? Sounds like a basic tuning issue.
Thread Starter
Racer
Joined: Oct 2005
Posts: 449
Likes: 0
From: Boulder CO
Originally Posted by lars
No, the outside air temp won't affect Q-Jet idle.
Why don't you bring the car over so we can take a look at it? Sounds like a basic tuning issue.
Why don't you bring the car over so we can take a look at it? Sounds like a basic tuning issue.
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Team Owner
Joined: Jun 2004
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From: Smile, it's just the internet.
Originally Posted by lars
No, the outside air temp won't affect Q-Jet idle.
Whenever I'm working on my 4150 carbed BB (heat on top of heat) with the hood up, the idle drops significantly. Once I put the hood down, it comes up several hundred RPM.
This is on a functional L88 induction system. Underhood temps are well into the triple digits. Never actually measured, but I can't touch anything w/o gloves or a rag. I have witnessed fuel boiling out of the vents from time to time. (I need to get a heat shield fabricated. The Holley shield won't fit and the 1/4" insulator gasket is too thick...pushes me into the hood.)
Instructor
Joined: Aug 2005
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From: St. George Ontario, Canada
Originally Posted by icanfixitforyou
Time to adjust the mixture screws for the warmer weather.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Living in Colorado and driving my car in temperatures from 20 degrees (with the top down) to 110 degrees I haven't had to touch an idle mixture screw or an idle speed screw on my Q-Jet or my BG. Idle stays stable and constant under all conditions. No adjustment required.
I have seen, however, that if a carb is jetted right on the ragged edge of the lean side, idle mixture screw adjustment becomes more sensitive. And if you have fuel boiling and/or vaporizing in the carb, you need to solve that little issue rather than monkey with the mixture screws... a heat shield is effective in controlling that problem (I run shields on all my stuff). If the carb is set up correctly and operating as it should, there should be no reason to have to continuously re-adjust mixtures and idle speed.
I have seen, however, that if a carb is jetted right on the ragged edge of the lean side, idle mixture screw adjustment becomes more sensitive. And if you have fuel boiling and/or vaporizing in the carb, you need to solve that little issue rather than monkey with the mixture screws... a heat shield is effective in controlling that problem (I run shields on all my stuff). If the carb is set up correctly and operating as it should, there should be no reason to have to continuously re-adjust mixtures and idle speed.
Team Owner
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From: Smile, it's just the internet.
Interesting.
Do you feel it's imperative to have an insulator gasket, or is a shield enough? I was going to fabricate something out of aluminum to fit over the studs on TOP of the carb base casting, so I didn't gain any height. I seriously have nothing to spare.
I ordered some header wrap on Monday to help with the heat, too.
I could gain a little height by trimming the vents and choke horn (which isn't functional anymore anyway), but I was always told never to trim the vents. I would like to mill the intake a bit, as the carb flange angle is wrong, but that's gonna have to wait till winter.
Do you feel it's imperative to have an insulator gasket, or is a shield enough? I was going to fabricate something out of aluminum to fit over the studs on TOP of the carb base casting, so I didn't gain any height. I seriously have nothing to spare.
I ordered some header wrap on Monday to help with the heat, too.
I could gain a little height by trimming the vents and choke horn (which isn't functional anymore anyway), but I was always told never to trim the vents. I would like to mill the intake a bit, as the carb flange angle is wrong, but that's gonna have to wait till winter.
Last edited by CGGorman; Apr 26, 2006 at 05:08 PM.