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308 carbs continued

Discussion in 'Technical Q&A' started by pcelenta, Nov 1, 2003.

  1. pcelenta

    pcelenta Karting

    Nov 1, 2003
    216
    continue of thread on 308 carbs....the last mechanic that serviced the carbs on my 78 did not believe in setting them up at idle, said that you don't drive your car at idle...any thoughts on this? since I rebuilt my carbs this past summer I have suffered the infamous 3500rpm transition stumble...never had this before.
     
  2. matteo

    matteo F1 World Champ

    Aug 1, 2002
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    Paul,


    My former Mechanic would set the carbs @ 2000 rpm's on the main's. Idle jets cut off @ 1800.
     
  3. pma1010

    pma1010 F1 Rookie
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    Jul 21, 2002
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    Matt
    I don't believe that's accurate. Pierce tells me idle jets will have an impact on the AF with the Webers all the way through a transition to mains at 3800 rpm or so.

    Some racers will sync the carbs at 5000 rpm. I am not sure I understand this in practice. I believe small differences in the throttle plates at idle will have a much larger impact on idle quality than small differences in the positions of the plates will have at 5000 rpm.
     
  4. mk e

    mk e F1 World Champ

    Oct 31, 2003
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    I'm pretty sure the jets are additive. So the idle jet supplies all or most or the fuel in the low rpms and part of the fuel the whole way up. The main jets probably begin supplying fuel about 2000 rpm and by 3000 or so are supplying most of the fuel, but the idle jets are still flowing as much fuel as they ever were, it just becomes a smaller and smaller percent of the total. That's way when jetting, you always, always, always get the idle right before trying to set the mains. If you try to do the mains first, you end up chasing your tail.
     
  5. Steve Magnusson

    Steve Magnusson F1 World Champ
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    Jan 11, 2001
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    After setting the ignition timing + carb airflow/mixture at idle, I think it's important to bring the individual barrel airflow up to ~7 kg/hr (using linkage in the same manner as the accelerator pedal cable would) and verify that the individual airflows are still reasonably matched (maybe something like +/- .25 ~ .5 kg/hr max difference) -- to make sure the linkage is all OK/syncronized, but my ex-'78 308 experience had no mid-RPM "stumbling" and I never needed to compromise one end for the other (and I'd say a street car spends a lot of time at idle).

    Have you done any intake vacuum line/system modifications from stock (e.g., is your stock airbox airflap stuff still intact and working well)?

    (And if you rebuilt the carbs why are you trusting someone else to set them up? ;) )
     
  6. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,353
    NY
    Set it up according to the book. It needs to run a little rich for it to pull all the way up to 8K. I don't have any problems going through 300 or 3500 RPM's with my 77. Make sure that the idle is set correctly and the carbs are balanced.
     
  7. Dave

    Dave F1 Rookie

    Apr 15, 2001
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    Steve,
    Unless your 308 engine has a good case of Mod work...
    every dyno run sheet I have ever seen shows a loss of HP in a stock 308 engine at that RPM... 8k
     
  8. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,353
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    With all of that whinning and sucking sounds you can't tell the difference.
     
  9. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
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    Idle jet "cutoff" is totally dependant upon airflow. At, say, 2500rpm, full throttle will use the main jet. However, a steady, part throttle cruise at 4000 is still totally on the idle jets, as there isn't much air flow. In addition, the throttle plate at "4000 cruise" is only just past the transition holes - which receive fuel entirely from the idles jets.
     
  10. pma1010

    pma1010 F1 Rookie
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    4re GT4
    Thanks for the clarification. Sounds right.
     

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