Lowering or increasing idle on carb 308 GTB | FerrariChat
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Lowering or increasing idle on carb 308 GTB

Discussion in '308/328' started by fredbrbr, Sep 25, 2025 at 1:32 PM.

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  1. fredbrbr

    fredbrbr Rookie

    Apr 21, 2023
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    Monaco
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    Fred Brouwers
    #1 fredbrbr, Sep 25, 2025 at 1:32 PM
    Hi everyone,
    I know that there are many threads on here on carb set up, idle adjustment etc... but I somehow cannot find an answer to my simple (maybe a little ignorant) question:

    The idle on my Ferrari 308 GTB has since recently been very low when cold (600rpm) and only goes up to something like 800rpm when hot, to the point that the car does die on me once in a while at idle. I'm aware that each carb has its own idle adjustment screw. However, is there a single idle screw for all 4 carbs together? If yes, where is this located? If not, in order to raise idle, should I adjust on each of all 4 carbs?

    Thanks in advance,

    Fred
     
  2. Sp1der71

    Sp1der71 Karting

    Jan 18, 2019
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    Simon Ashley
    #2 Sp1der71, Sep 25, 2025 at 2:05 PM
  3. fredbrbr

    fredbrbr Rookie

    Apr 21, 2023
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    Fred Brouwers
    #3 fredbrbr, Sep 25, 2025 at 3:04 PM
  4. bitsobrits

    bitsobrits Formula Junior
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    Nov 12, 2011
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    Omaha, NE area, US
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    Steve
    #4 bitsobrits, Sep 25, 2025 at 3:06 PM
    Last edited: Sep 25, 2025 at 3:21 PM
    You should read those tutorials more closely. The 246 guide will not help you.

    The following applies only for the hot idle speed. For the cold idle speed, you would want to adjust the cold idle unit, which on my car is not active, so I can't help you with that.

    There is no single screw, and the four carbs do not have individual idle speed screws. There are two throttle stop screws, one for each cylinder bank, on the two left hand carbs as you face the engine from the rear, and not to be confused with the L/R balancing screws between the carbs. See the photo for step 6 which is taken from Birdmans' carb tuning tutorial.

    Note that the forward throttle stop screw is extremely hard to see and adjust with the air cleaner installed. It can be done by feel with a small or stubby screwdriver once you are able to locate it.

    FWIW it doesn't take much in the way of twiddling the wrong screws to mess things up. You need to make sure to locate the two throttle stop screws. To raise the idle speed a couple of hundred rpm should only take turning the screws in (clockwise) about 1/16 turn, maybe less.
     

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  5. ChevyDave

    ChevyDave Formula Junior

    Dec 21, 2019
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    Dave W
    #5 ChevyDave, Sep 26, 2025 at 3:51 PM
    Technically, there is a "one-screw adjustment" for idle speed (assuming the inter-carb linkages are adjusted properly): the throttle cable adjuster at the front cam cover.
    Obviously not a proper way to set idle speed, but that said, the throttle cable adjuster can help properly adjust your throttle stop screws (assuming the low idle condition is being caused by improperly set throttle stop screws). Similar to Steps 5 & 8 in Birdman's tutorial, you can increase idle speed using the throttle cable adjuster to the 900-1,000 RPM range, then bring your Throttle Stop Screws to the point they are just touching the linkage. Once your Throttle Stop Screws are controlling the idle speed, back-off the throttle cable adjuster so it has just the slightest bit of slack.
    The other thing that's gone unsaid here though is why did your idle speed drop in the first place? That's probably something worth exploring.
    - Dave
     

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