Am I chasing a ghost re: idle speed?
Thread Starter
Race Director
Joined: Jan 2000
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From: San Jose CA
Am I chasing a ghost re: idle speed?
My 78 (manual, L-82 carb) runs at a neutral idle of about 850 RPM. If I pull the vac advance that will drop it down to about 625. Base timing is 12*.
I have no adjustment left in the idle speed screw. My emissions sticker says the idle should be 700 RPM with vac advance disconnected, since I'm well below that should I be happy and go on with life?
When I had the auto, it would idle a bit lower but I've also done a lot of work to make this thing run better so this isn't exactly science.
I have no adjustment left in the idle speed screw. My emissions sticker says the idle should be 700 RPM with vac advance disconnected, since I'm well below that should I be happy and go on with life?
When I had the auto, it would idle a bit lower but I've also done a lot of work to make this thing run better so this isn't exactly science.
Race Director
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I could be wrong but on an emissions control car you should not have vacuum advance at idle. Is your car modified that you have manifold vacuum at idle as opposed to ported vacuum from the appropriate carburetor connection? I assume you have a quadrajet?
Le Mans Master
Joined: Oct 2006
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From: El Cajon California
I have the same issue with my 1978 Vette, tach does not work so I dont know the idle speed but I know it is a bit high when I run 12-14 degrees advance, when backed down to 6 idle seems about right. I don't believe I have a vacuum leak I checked using carb cleaner and pulling a steady vacuum, i think it was about 20. Carb was a Lars rebuild so I know the carb is ok.
I will be following this thread
I will be following this thread
Thread Starter
Race Director
Joined: Jan 2000
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From: San Jose CA
Yes, it's a 17058228(technically L-82 auto) QJet, and I'm running off the correct port (above left side(driver) idle mixture screw), but I still get some light vacuum on that port. I agree with your assessment, I shouldn't be getting any.
Le Mans Master
Joined: Jul 2006
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From: charlotte north carolina
since it will idle at 625 with the vacuum disconnected, but jumps to 850 with the vacuum connected, i would think that the vacuum advance can is wrong for the manual trans or the port vacuum is wrong. the engine may be getting too much vacuum advance at idle.
Thread Starter
Race Director
Joined: Jan 2000
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From: San Jose CA
jnb5101 - I've been wondering the same. I need to learn a bit about the ins and outs of picking a vacuum can.
Le Mans Master
Joined: Jul 2006
Posts: 8,117
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From: charlotte north carolina
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Race Director
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From: Sierra Foothills CA
Steve, confirm you're running the HEI?
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From: Sierra Foothills CA
Thread Starter
Race Director
Joined: Jan 2000
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From: San Jose CA
I just did some research and looked at what I've got - it's an AR-12 unit as provided by Lars a number of years back. I'll take some measurements on the port I'm using vs some of the manifold vac ports and see how they behave at idle, higher RPM etc
Thread Starter
Race Director
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From: San Jose CA
So the port I was using is the correct port as near as I can tell.
The port just above the driver's side idle mixture screw, bottom center of photo.
It is definitely a manifold vacuum port. I got 15in-hg roughly off the port (same as any other manifold vacuum port, with vac advance disconnected) and it raised in response to throttle.
I pulled the EGR vacuum port and that port definitely behaves differently, it gets zero vacuum at idle and continues to stay near zero until I crack the throttle quite a ways.
However, all the diagrams I have for the carb show that I have the vac advance hooked up correctly.
I'm starting to really feel like I'm chasing ghosts at this point.
The port just above the driver's side idle mixture screw, bottom center of photo.
It is definitely a manifold vacuum port. I got 15in-hg roughly off the port (same as any other manifold vacuum port, with vac advance disconnected) and it raised in response to throttle.
I pulled the EGR vacuum port and that port definitely behaves differently, it gets zero vacuum at idle and continues to stay near zero until I crack the throttle quite a ways.
However, all the diagrams I have for the carb show that I have the vac advance hooked up correctly.
I'm starting to really feel like I'm chasing ghosts at this point.