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308 GT4- more carb questions

Discussion in 'Technical Q&A' started by Doug, Apr 1, 2004.

  1. Doug

    Doug Formula 3

    Nov 13, 2003
    1,467
    Louisville KY
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    Doug
    I am slowly trying to dial in my '76 308 GT4. Car has a single dist. conversion with a Mallory Unilite set-up. I have adjusted the advance to spec. Should this be different than spec because of the conversion set up? The carbs have 135 mains and 45 idle jets.

    1. I get popping/crackling when I let off the throttle. Research tells me that this could be too lean and so I have ordered some 50 and 55 idles.

    2. The car is slow to drop back to idle. When I accelerate and shift, the rpm doesn't drop when I press in the clutch. It's more annoying than anything.
    Any suggestions?
    Doug
     
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  3. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    19,741
    Texas
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    Steve Magnusson
    If you can confirm that the throttle plates are closing well, #2 might be related to the ignition side of things. Are you able to get the same bad effect when stationary using the throttle to bring-up and then drop the RPM?

    Please give more information and you might get better input:

    Does your current carb plumbing set match the specifications as shown in your year/version OM (or in what ways doesn't it)?

    What ignition "spec" are you set to at idle? I believe this would depend on which advance curve (S159A or S159B) that your new ignition set-up better matched (the "A" set-up gives more advance in the idle-to-1800 RPM range, but they're about the same in the 1800 RPM and up range -- not saying that I know one is definitely better than the other for you, but you've got to take the advance curve into account to select a reasonable idle setting range). Do you have the advance curve for your new set-up?

    What does the Synchrometer read at idle?
     
  4. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    2,559
    Chicago
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    Philip
    It'll run better with 55 idles.

    Recommend you put them in, re-sync and post the readings.

    RPM can be slow to return to idle when (i) rich [not likely]; (ii) distrib is "slow" to react [unknown]; (iii) throttle shafts are worn; (iv) return springs are suspect or linkage is binding on something.
    Philip
     
  5. Doug

    Doug Formula 3

    Nov 13, 2003
    1,467
    Louisville KY
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    Doug
    Advance is 7deg @ 1000 and I am using one of those digital scopes that measures rpm off the plug wire. The service manual I got with the car says 130 mains and 45 idles. I dropped in the 135s because the extra set came with the car. From what I read, most are upping the idles to 50-60 so I ordered 50 & 55's today. Things are getting better as I tinker with it. I guess I will wait until I get different idle jets to see what happens before messing with it any more. It starts and runs great, just pops/crackles on deceleration. Plugs are all looking good. The sticking RPM is dropping a little faster as I keep messing with it.
     
  6. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
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    Hans E. Hansen
    I'm surprised that it runs with 45 idles.

    I've experimented a bit with idle jets in an attempt to burn cleaner and improve gas mileage. But I get 'bad behaviours' with anything less than
    55s.
     
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  8. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    19,741
    Texas
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    Steve Magnusson
    Doug -- I'd still recommend that you go by the "carb setting data" in the OM for your exact year/model/version as the starting/reference point (since this has a chance of getting updated whereas service information may not). The table in the Haynes Weber book shows the 35/36/37/38 series 40DCNF using 45 idle jets in the stock configuration (but that series corresponds to an earlier-than-yours, non-US 308GT4). Later non-US, US, and AUS 308GT4 all use the larger idle jets so doesn't seem to be an unreasonable direction to try.

    Which series 40DCNF do you have? 35/36/37/38, 45/46/47/48, 57/58/59/60, or something else?
     

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