Carb Tuning with Advanced Camshaft
Thread Starter
Racer
Joined: Jun 2017
Posts: 495
Likes: 85
Carb Tuning with Advanced Camshaft
Hey everyone,
I have kind of a high level question for you all.
We know the ignition timing plays a huge difference in carb tuning, but I can't find much info on advancing or retarding the camshaft.
I rebuilt my motor a few years ago, and it runs pretty good, but runs rich at idle (12.0 afr), just off idle it gets a bit lean (14.2-14.19 afr), and is good on anything off idle (13.2 to 13.9 afr). When I rebuilt the motor I put in a pretty mild retro-roller cam and advanced it 2 degrees. Before I rebuilt the motor, it created good vacuum, about 17 inches at idle. Once I rebuilt the motor and advanced the camshaft, the motor makes even higher vacuum, about 19-22 inches.
Generally when the a motor makes less vacuum at idle, it's recommended to increase jet sizes...
Could the advanced camshaft be complicating my carb tuning?
I have kind of a high level question for you all.
We know the ignition timing plays a huge difference in carb tuning, but I can't find much info on advancing or retarding the camshaft.
I rebuilt my motor a few years ago, and it runs pretty good, but runs rich at idle (12.0 afr), just off idle it gets a bit lean (14.2-14.19 afr), and is good on anything off idle (13.2 to 13.9 afr). When I rebuilt the motor I put in a pretty mild retro-roller cam and advanced it 2 degrees. Before I rebuilt the motor, it created good vacuum, about 17 inches at idle. Once I rebuilt the motor and advanced the camshaft, the motor makes even higher vacuum, about 19-22 inches.
Generally when the a motor makes less vacuum at idle, it's recommended to increase jet sizes...
Could the advanced camshaft be complicating my carb tuning?
Burning Brakes
Joined: Dec 2017
Posts: 1,043
Likes: 414
2023 C3 of the Year Finalist - Modified
Advancing a cam will raise your DCR and improve low end response.
I don’t think you need to change your carb jets just from advancing the cam 2 degrees. the intake valve is just shutting sooner.
I think the best way to pick jet sizes is an O2 sensor to check your AFR.
Gather data for part throttle pulls and WOT pulls and then adjust from there
I don’t think you need to change your carb jets just from advancing the cam 2 degrees. the intake valve is just shutting sooner.
I think the best way to pick jet sizes is an O2 sensor to check your AFR.
Gather data for part throttle pulls and WOT pulls and then adjust from there
Last edited by randallsteel; Aug 23, 2023 at 07:32 PM.
Thread Starter
Racer
Joined: Jun 2017
Posts: 495
Likes: 85
yep, can't agree more. I have an AFR gauge in my car and that's how i got the readings noted above.
I was just thinking if the motor creates more vacuum at idle, would you increase air bleeds, or decrease the idle fuel restriction
I was just thinking if the motor creates more vacuum at idle, would you increase air bleeds, or decrease the idle fuel restriction
Race Director
Joined: Jan 2000
Posts: 13,024
Likes: 2,266
From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
2020 Corvette of the Year (performance mods)
C2 of Year Winner (performance mods) 2019
2017 C2 of Year Finalist
It's not the advanced position...it's the cam change itself to a mild one holding that much vacuum. Not sure what carb you have...but assuming a typical Holley?
With those readings I'd work on the idle circuit to lean it. You could increase the idle air bleeds first as an easy change...but understand it might make that initial off idle lean spot a little worse....but you can tweak that with accelerator pump and/or PV adjustments/changes. You can also decrease the idle fuel feed restrictions...but that's more involved. Changing the jets to smaller isn't going to change idle behavior.
JIM
With those readings I'd work on the idle circuit to lean it. You could increase the idle air bleeds first as an easy change...but understand it might make that initial off idle lean spot a little worse....but you can tweak that with accelerator pump and/or PV adjustments/changes. You can also decrease the idle fuel feed restrictions...but that's more involved. Changing the jets to smaller isn't going to change idle behavior.
JIM
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Unless you are class racing......there is zero benefit to advancing and retarding a cam as it yields very little change.....and I generally advice against it because it is easy to screw things up if one is not confident with a degree wheel.
Tuning is determined by vacuum, swept volume, velocity and RPM......as well as heat and density.....moving the cam has very little play here.
Jebby
Tuning is determined by vacuum, swept volume, velocity and RPM......as well as heat and density.....moving the cam has very little play here.
Jebby
Thread Starter
Racer
Joined: Jun 2017
Posts: 495
Likes: 85
Unless you are class racing......there is zero benefit to advancing and retarding a cam as it yields very little change.....and I generally advice against it because it is easy to screw things up if one is not confident with a degree wheel.
Tuning is determined by vacuum, swept volume, velocity and RPM......as well as heat and density.....moving the cam has very little play here.
Jebby
Tuning is determined by vacuum, swept volume, velocity and RPM......as well as heat and density.....moving the cam has very little play here.
Jebby
I really appreaciate your advice on this topic.
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Thread Starter
Racer
Joined: Jun 2017
Posts: 495
Likes: 85
Right now I have them about 9/16ths of a turn out right now, and it's really rich at idle. If i go 1/2 turn, it goes quite lean and off-idle gets too lean (like 15.2-15.5 afr).
Currently, even though off-idle is a bit lean, idle is really rich. I tried to remedy this by opening up the idle fuel restriction just a tad, but this makes the idle screws even more sensitive.
I tried opening up the idle air bleed a bit, but this made the idle to off-idle afr swings even worse.
The car feels good, but would just appreciate a more consistent AFR across idle, off-idle, and 10-100% throttle.
This is all on a 650 spreadbore holley.
I have a q-jet, and it has 'idle air bypass' orfice which would help with this situation. I guess i could start drilling the throttle blades on my holley, but from what I've read, that is typically done with not enough vacuum at idle
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
My idle mixture screws are EXTREMELY sensitive, and I'm not sure why. A guess is maybe higher vacuum at idle.
Right now I have them about 9/16ths of a turn out right now, and it's really rich at idle. If i go 1/2 turn, it goes quite lean and off-idle gets too lean (like 15.2-15.5 afr).
Currently, even though off-idle is a bit lean, idle is really rich. I tried to remedy this by opening up the idle fuel restriction just a tad, but this makes the idle screws even more sensitive.
I tried opening up the idle air bleed a bit, but this made the idle to off-idle afr swings even worse.
The car feels good, but would just appreciate a more consistent AFR across idle, off-idle, and 10-100% throttle.
This is all on a 650 spreadbore holley.
I have a q-jet, and it has 'idle air bypass' orfice which would help with this situation. I guess i could start drilling the throttle blades on my holley, but from what I've read, that is typically done with not enough vacuum at idle
Right now I have them about 9/16ths of a turn out right now, and it's really rich at idle. If i go 1/2 turn, it goes quite lean and off-idle gets too lean (like 15.2-15.5 afr).
Currently, even though off-idle is a bit lean, idle is really rich. I tried to remedy this by opening up the idle fuel restriction just a tad, but this makes the idle screws even more sensitive.
I tried opening up the idle air bleed a bit, but this made the idle to off-idle afr swings even worse.
The car feels good, but would just appreciate a more consistent AFR across idle, off-idle, and 10-100% throttle.
This is all on a 650 spreadbore holley.
I have a q-jet, and it has 'idle air bypass' orfice which would help with this situation. I guess i could start drilling the throttle blades on my holley, but from what I've read, that is typically done with not enough vacuum at idle
Jebby
Thread Starter
Racer
Joined: Jun 2017
Posts: 495
Likes: 85
Currently, my primary idle setting is really low, like 1/3 turn past touching the throttle lever. Transfer slot isn't square, it's less than square would be. Like a horizontal rectangle.
Race Director
Joined: May 2006
Posts: 18,771
Likes: 2,596
yes, i tried playing with secondary idle stop for more air at idle to help with increasing idle AFR. If I opened it enough, it worked at idle in my driveway, but as soon as I drove it around, and say I was coasting to a red light in gear at 1500rpm it would be super lean, so I put it back to rich idle to avoid detonation.
Currently, my primary idle setting is really low, like 1/3 turn past touching the throttle lever. Transfer slot isn't square, it's less than square would be. Like a horizontal rectangle.
Currently, my primary idle setting is really low, like 1/3 turn past touching the throttle lever. Transfer slot isn't square, it's less than square would be. Like a horizontal rectangle.