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  •  » Tuned my new ATM carb

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    5/31/2026 7:25 AM  #1

    RTM
    Senior 24-7 FYI Addict

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    From: Cape Coral FL
    Registered: 5/22/2019
    Posts: 2,095

    Tuned my new ATM carb

    Looking back on all my years of running carbs I've never really dug into the finer things of tuning them.  Maybe I was always lucky because they all ran well and started right up.  Some nice I built this motor I just guessed it was hard to start because there was no choke.  I also had trouble driving at slow speeds low rpm.   Again I just thought it was the cam profile causing this.  I just always adjusted the idle based on vacuum.  Of course I played with jet sizes but not on this build.   

    With all of this I figured I would send my carb off to get custom tuned for my setup. After researching costs I decided on a new custom tuned carb from ATM.  My old carb I bought off marketplace.   Even with the new carb I did the vacuum tune and still had all the same issues.  What made me look deeper into this is I drove it on one of longest drives and late in the drive I would go to pull away from a stop and it would act like it was flooded. I made it to where I was going and all seemed fine by the time I got there.  The drive home it ran perfect.  Even started right up when I left. 

    I found a topic on how to adjust a DP carb for better drivability so I finally took the time to try this approach out.  I've never heard of this approach before. Close front butterfly's and set idle to 1000 rpm with the secondary butterfly's.  Then set from to idle at 1400 rpm.  Now rev the engine to 2100-2300 rpm.  If the rpm hovers above 1400 before returning to 1400 it's lean.  If it drops below 1400 it's rich.  I got that dialed in by making small adjustments to each idle circuit, same adjustment on each corner.  Once I felt I was good you then drop the idle by 100 rpm starting with the secondary then primary.  When I got closer to my desired idle rpm I adjusted in 50 rpm increments. 

    It's night and day difference in start ups and drivability.   I've never claimed to be a carb expert and I never will but I sure did learn something on this one.  Keep in mind  only had 200 miles on the mustang since I finished it.  So I really hadn't driven it much being so busy with other stuff.  Now I want to drive it everywhere!

     

    5/31/2026 11:22 AM  #2

    6sally6
    Hyper FYIFORD ADDICT

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    From: Surfside Beach,SC
    Registered: 4/02/2013
    Posts: 7,008

    Re: Tuned my new ATM carb

    Always 'heard' the 4-corner adjustment ability on some carbs really made tuning them a lot better.
    Interesting about the 1400 RPM deal !
    6sally6

    Get busy Liv'in or get busy Die'n....Host of the 2020 Bash at the Beach/The only Bash that got cancelled  )8
     

    5/31/2026 3:59 PM  #3

    RTM
    Senior 24-7 FYI Addict

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    From: Cape Coral FL
    Registered: 5/22/2019
    Posts: 2,095

    Re: Tuned my new ATM carb

    6sally6 wrote:

    Always 'heard' the 4-corner adjustment ability on some carbs really made tuning them a lot better.
    Interesting about the 1400 RPM deal !
    6sally6

     
    It's amazing how much better it starts, idles and cruises.  I don't have to rev the engine so much on take off now.   I've always closed the rear and adjusted from there.  I'm so excited and want to drive it everywhere now.  Lol

       •Thread Starter
     

    5/31/2026 4:29 PM  #4

    Nos681
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    From: Atlantic Mine, Michigan
    Registered: 7/14/2016
    Posts: 3,560

    Re: Tuned my new ATM carb

    Hey Bob, mind sharing that article ewe bee tawkin’ ‘bout?

     

    6/01/2026 7:41 AM  #5

    RTM
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    From: Cape Coral FL
    Registered: 5/22/2019
    Posts: 2,095

    Re: Tuned my new ATM carb

    I copied this from the Facebook post. 

    Start your car and get it up to operating temperature of at least 180°
    Then close the primary throttle blade completely shut, and set the secondary throttle blade to 1000 RPM.
    Go back to the primary throttle blade add another 400 RPM so that the car is now at a 1400 RPM idle.
    With the car idling at 1400 RPM give it a real quick blip to 2300rpm, no more than 2500 but no less than 2100. If you miss it, give it a few seconds and try again.
    As you do this quick blip, watch the tachometer.
    We want to go 1400–2300-then back to 1400 exactly .
    If it hangs higher than 1400, that means it’s lean and you need to come out on the mixture screws
    If it drops lower than 1400, It means it’s fat and you need to go in on the mixture screws
       Or goal is 1400–2300 –1400 every single time.

    Make very small adjustments on your mixture screws as you’re doing this, once we get to go 1400–2300–1400 we will then back to primary screw out again, reset the secondary to 1000 RPM readjust the front one to 1400 again if you go to this whole process one more time with making smaller adjustments on the mixture screws, screwdriver thickness is about right for the second round.

    Once we have this set properly. You’ll be able to walk away from the car come back five minutes later reach in the window,  hit the key and it will start without touching the gas.

    Do Not, Do Not set your idle using a wide band.

    There are a lot of things that can effect a wide band reading , not just a carb adjustment.
       Make sure that you have the correct spark plugs and ignition timing as they will cause a difference in the reading.

    The wide band is a gauge, and needs to be used as a gauge. Period.
    It is used to watch trends in Performance, much like an oil pressure gauge or water temperature gauge.

       •Thread Starter
     

    6/01/2026 11:49 AM  #6

    Steve69
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    From: Madison Wisconsin
    Registered: 4/01/2013
    Posts: 2,819

    Re: Tuned my new ATM carb

    RTM wrote:

    I copied this from the Facebook post.

    Start your car and get it up to operating temperature of at least 180°
    Then close the primary throttle blade completely shut, and set the secondary throttle blade to 1000 RPM.
    Go back to the primary throttle blade add another 400 RPM so that the car is now at a 1400 RPM idle.
    With the car idling at 1400 RPM give it a real quick blip to 2300rpm, no more than 2500 but no less than 2100. If you miss it, give it a few seconds and try again.
    As you do this quick blip, watch the tachometer.
    We want to go 1400–2300-then back to 1400 exactly .  
    If it hangs higher than 1400, that means it’s lean and you need to come out on the mixture screws
    If it drops lower than 1400, It means it’s fat and you need to go in on the mixture screws
    Or goal is 1400–2300 –1400 every single time.

    Make very small adjustments on your mixture screws as you’re doing this, once we get to go 1400–2300–1400 we will then back to primary screw out again, reset the secondary to 1000 RPM readjust the front one to 1400 again if you go to this whole process one more time with making smaller adjustments on the mixture screws, screwdriver thickness is about right for the second round.

    Once we have this set properly. You’ll be able to walk away from the car come back five minutes later reach in the window, hit the key and it will start without touching the gas.

    Do Not, Do Not set your idle using a wide band.

    There are a lot of things that can effect a wide band reading , not just a carb adjustment.
    Make sure that you have the correct spark plugs and ignition timing as they will cause a difference in the reading.

    The wide band is a gauge, and needs to be used as a gauge. Period.
    It is used to watch trends in Performance, much like an oil pressure gauge or water temperature gauge.

       Can you adjust the Secondary blades on a Vacuum secondary carburetor?   Any youtube video's on this process.  
    Thanks for posting!
     

     

    6/01/2026 4:55 PM  #7

    RTM
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    From: Cape Coral FL
    Registered: 5/22/2019
    Posts: 2,095

    Re: Tuned my new ATM carb

    I can only assume this process is for DP carburetor but I'm not that knowledgeable on carburetors.  I don't think you can adjust the rear butterfly on a vacuum carb?

    I still can't get over how well my car starts hot or cold and drives freaking amazing.

       •Thread Starter
     

    6/01/2026 9:05 PM  #8

    Texas!
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    Registered: 6/27/2014
    Posts: 238

    Re: Tuned my new ATM carb

       Can you adjust the Secondary blades on a Vacuum secondary carburetor?  
     

    The adjustment screw is on the bottom of the carb. You have to remove the carb and turn it over to access the screw.

     

    Yesterday 2:28 AM  #9

    Nos681
    Senior 24-7 FYI Addict

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    From: Atlantic Mine, Michigan
    Registered: 7/14/2016
    Posts: 3,560

    Re: Tuned my new ATM carb

    Thanks Bob!

    Do not recall reading about this procedure before.

    Mark Sullens?

    Last edited by Nos681 (Yesterday 3:15 AM)

     

    Yesterday 5:35 AM  #10

    RTM
    Senior 24-7 FYI Addict

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    From: Cape Coral FL
    Registered: 5/22/2019
    Posts: 2,095

    Re: Tuned my new ATM carb

    Steve69 wrote:

    RTM wrote:

    I copied this from the Facebook post.

    Start your car and get it up to operating temperature of at least 180°
    Then close the primary throttle blade completely shut, and set the secondary throttle blade to 1000 RPM.
    Go back to the primary throttle blade add another 400 RPM so that the car is now at a 1400 RPM idle.
    With the car idling at 1400 RPM give it a real quick blip to 2300rpm, no more than 2500 but no less than 2100. If you miss it, give it a few seconds and try again.
    As you do this quick blip, watch the tachometer.
    We want to go 1400–2300-then back to 1400 exactly .  
    If it hangs higher than 1400, that means it’s lean and you need to come out on the mixture screws
    If it drops lower than 1400, It means it’s fat and you need to go in on the mixture screws
    Or goal is 1400–2300 –1400 every single time.

    Make very small adjustments on your mixture screws as you’re doing this, once we get to go 1400–2300–1400 we will then back to primary screw out again, reset the secondary to 1000 RPM readjust the front one to 1400 again if you go to this whole process one more time with making smaller adjustments on the mixture screws, screwdriver thickness is about right for the second round.

    Once we have this set properly. You’ll be able to walk away from the car come back five minutes later reach in the window, hit the key and it will start without touching the gas.

    Do Not, Do Not set your idle using a wide band.

    There are a lot of things that can effect a wide band reading , not just a carb adjustment.
    Make sure that you have the correct spark plugs and ignition timing as they will cause a difference in the reading.

    The wide band is a gauge, and needs to be used as a gauge. Period.
    It is used to watch trends in Performance, much like an oil pressure gauge or water temperature gauge.

       Can you adjust the Secondary blades on a Vacuum secondary carburetor?   Any youtube video's on this process.  
    Thanks for posting!
     

     
    I do not know that answer to the ATM carb but it looks like it could be.  Not sure how that impact the procedure I followed.  Again I'm far from a carb expert.  Still learning the programming of my timing table which I need to fine tune next per instructions.

       •Thread Starter
     

    Yesterday 2:46 PM  #11

    Steve69
    Senior 24-7 FYI Addict

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    From: Madison Wisconsin
    Registered: 4/01/2013
    Posts: 2,819

    Re: Tuned my new ATM carb

    RTM wrote:

    Steve69 wrote:

    RTM wrote:

    I copied this from the Facebook post.

    Start your car and get it up to operating temperature of at least 180°
    Then close the primary throttle blade completely shut, and set the secondary throttle blade to 1000 RPM.
    Go back to the primary throttle blade add another 400 RPM so that the car is now at a 1400 RPM idle.
    With the car idling at 1400 RPM give it a real quick blip to 2300rpm, no more than 2500 but no less than 2100. If you miss it, give it a few seconds and try again.
    As you do this quick blip, watch the tachometer.
    We want to go 1400–2300-then back to 1400 exactly .  
    If it hangs higher than 1400, that means it’s lean and you need to come out on the mixture screws
    If it drops lower than 1400, It means it’s fat and you need to go in on the mixture screws
    Or goal is 1400–2300 –1400 every single time.

    Make very small adjustments on your mixture screws as you’re doing this, once we get to go 1400–2300–1400 we will then back to primary screw out again, reset the secondary to 1000 RPM readjust the front one to 1400 again if you go to this whole process one more time with making smaller adjustments on the mixture screws, screwdriver thickness is about right for the second round.

    Once we have this set properly. You’ll be able to walk away from the car come back five minutes later reach in the window, hit the key and it will start without touching the gas.

    Do Not, Do Not set your idle using a wide band.

    There are a lot of things that can effect a wide band reading , not just a carb adjustment.
    Make sure that you have the correct spark plugs and ignition timing as they will cause a difference in the reading.

    The wide band is a gauge, and needs to be used as a gauge. Period.
    It is used to watch trends in Performance, much like an oil pressure gauge or water temperature gauge.

       Can you adjust the Secondary blades on a Vacuum secondary carburetor?   Any youtube video's on this process.  
    Thanks for posting!
     

     
    I do not know that answer to the ATM carb but it looks like it could be. Not sure how that impact the procedure I followed. Again I'm far from a carb expert. Still learning the programming of my timing table which I need to fine tune next per instructions.

      Thanks for the info!
     

     

    Yesterday 2:47 PM  #12

    Steve69
    Senior 24-7 FYI Addict

    Offline

    From: Madison Wisconsin
    Registered: 4/01/2013
    Posts: 2,819

    Re: Tuned my new ATM carb

    Texas! wrote:

       Can you adjust the Secondary blades on a Vacuum secondary carburetor?  
     

    The adjustment screw is on the bottom of the carb. You have to remove the carb and turn it over to access the screw.

      Thanks for the info. That would be hard when its running...lol  
     

     

    Today 8:08 AM  #13

    BobE
    Senior 24-7 FYI Addict

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    From: White Plains, NY
    Registered: 4/04/2013
    Posts: 3,002

    Re: Tuned my new ATM carb

    RTM – regarding adjusting the secondary throttle blades.  On my Holley 650 DP with an Edelbrock RPM Performer manifold, I was able to adjust the blades w/o removing carburetor.  I used an Xecelite screwdriver #R181N, it has 1/8” wide tip with a 2” shaft and comes with a hex handle that can be turned with a ½” wrench.  I did have to cut off some of the handle to allow it to fit in that area. 
    I would send a photo of this modified screwdriver but I cannot find it among my tools.

    65 Fastback, 351W, 5-speed, 4 wheel discs, 9" rear,  R&C Front End.
     

     

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