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Jetting for a 308/GT4 in New England

Discussion in '308/328' started by Erol, May 2, 2021.

  1. Erol

    Erol Karting
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    Sep 1, 2020
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    Erol
    Hi all

    Have read through a bunch of threads on here re jetting my 1975 GT4 and talked to a few folks...I think I have a plan.

    My jetting is stock other than idle jet where I went from 55 to 65 (perhaps too much but I had a real dead spot around 3500 that I was told a bigger idle jet would help address).

    Hesitation in that transition area is gone but now once the car is at temperature, I’m having much worse hesitation and bogging to the point of wanting to stall while on the idle jet - particularly from a full stop until around 3-4k when suddenly I get on the main and take off...needless to say, makes getting across intersections rather harrowing! Am considering going back to the 55 for idle and rest as follows:

    130 main
    F 36 tubes
    200 AC
    55 idle
    Set timing @ 10*

    Anyone else running a setup like this?

    I’m in NYC so ethanol free isn’t an option and I typically go with 93 (also read that 91 could be preferable if anyone has an opinion there?)

    Also imagine many of you are already familiar with this write up but for those who aren’t, I really learned a lot here:
    http://dino246blog.blogspot.com/2014/08/carb-tuning-1-weber-dcnf-carbs-and.html?m=1
    And this is great:
    http://www.teglerizer.com/dcoe/inglese/inglesetuning.htm

    Thanks
     
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  3. 4right

    4right Formula 3
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    I just went threw a similar situation with my 308 GTB. Lots of hesitation low in the rpm range especially when trying to accelerate from a full stop. Every time I went anywhere near the accelerator I would also get a series of sneezes. I am now running 140 main jets, 185 air correctors, (200 made the situation worse, leaned it out too much), 70 idle jets, F36 emulsion tubes. The sneezes have completely gone away and the transition is smooth from idles to main jets. I am still getting a slight stumble from a full stop but nothing terrible. The floats were also adjusted, all four were at different setting.
     
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  4. Erol

    Erol Karting
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    Sep 1, 2020
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    Erol
    Wow 70 for the idle jets seems big! I thought I had gone too high w 65. How do your plugs look?
     
  5. 4right

    4right Formula 3
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    I have only driven the car twice since these adjustments were made. So far so good! Oh, installed new extenders which helped as well.
     
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  6. Ferraripilot

    Ferraripilot F1 World Champ
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    May 10, 2006
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    70 idles is really huge, what's going on there!? I've set up a few early cam engines and they really like a 140 main with between 190-200 air corrector. Start with 140 mains see how she goes. Additionally, make sure you're getting full ignition timing at 6000rpm, should be about 37-38 degrees all-in.
     
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  8. Erol

    Erol Karting
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    Super helpful re timing thank you.

    Any idea what you typically did for idles with the140 mains?

    I had 135 mains, 220 air correctors and 55 idles before swapping the 65 idles...didn’t note the emulsion tubes but folks don’t seem to mention them so not sure how important they are or what priority they have. Wondering if maybe best to go to 55 idles and the 140 mains like you suggested before I go nuts w AC and emulsion tubes?

    Like I said, with my old setup acceleration would kind of fall flat at around 3500. Was told a larger idle would help with the transition to the main jet and it worked to the extent that it pulls strong all the way through but now once warm, hesitation is at low rpms and when pulling away from a full stop.

    thanks again
     
  9. 4right

    4right Formula 3
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    Not really sure, however, it would constantly sneeze just sitting there idling. While driving, every time you touched the gas pedal it would result in a flurry of sneezes. New extenders, adjusted floats and 70 idles and no more sneezing.
     
  10. BigTex

    BigTex Seven Time F1 World Champ
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    There's a chart "on here" with OEM jetting and these guys posting have good knowledge of updates for modern fuel.

    I would also add in that ignition advance is HUGE.....did I say really important?....HUGE.
    So a calibration and lube of both distributors is actually a 15k service on these cars.
     
  11. Ferraripilot

    Ferraripilot F1 World Champ
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    A larger main brings the main circuit in a bit earlier, even more so with a small air corrector as well. I have always landed at no larger than a 60 idle jet, but as we can see above, 4right had to go considerably larger although that is the only circumstance I have heard of that. I'm wondering if he has DCNFs with different progression holes, as the US spec, Euro spec, and general retail spec all had slightly different idle circuit progression holes.
     
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  13. ProvaMo

    ProvaMo Karting
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    #10 ProvaMo, May 4, 2021
    Last edited: May 4, 2021
    Erol- Good thread, link below is a good f-chat 308 Carb source (Post #4 has the chart BigTex mentions, and #12 is my compilation of good jetting threads):
    https://www.ferrarichat.com/forum/threads/euro-carbs-conversion.634518/#post-147648091

    I recently rebuilt and rejetted my 1980 Euro GTB carbs, and will be installing them in the coming two weeks, FWIW, I'm running 100LL AVGAS. Looking forward to road testing and reporting back.
    This is the jetting:
    210 Air Correction
    F36 Emulsion Tubes
    135 Main Jets
    55 Idle Jets
     
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  14. 4right

    4right Formula 3
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    I am running high compression pistons in a 76 Euro 308. Not sure if that plays a role in needing 70 idle jets to get the engine running correctly. The pistons are Daytona Comp equivalent, the one pictured on the left.

    Image Unavailable, Please Login
     
  15. ProvaMo

    ProvaMo Karting
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    Paul John
  16. newto308

    newto308 Karting

    Mar 20, 2007
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    Chris
    I'd be looking to put an O2 bung in each bank and connect a wide-band O2 sensor in each side. This is the only way you rare going to know where the lean/rich spots are through the RPM range for any given Main, ET and Air Correction. I'm not running stock cams and I have high-flow intake vales. I was really surprised to find a big lean spot between carb circuits.
     

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