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Need help with engine stumbling on carb 308

Discussion in 'Technical Q&A' started by jpl, Jul 4, 2004.

  1. jpl

    jpl Formula Junior

    Dec 4, 2003
    322
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    JP Lavigne
    First the facts.
    1978 308GTB
    Using crane XR700's instead of points etc

    Recent carb work done (cleaned and rebuilt at dealer with appropriate kits)

    Now the problem.

    The car has been hesitating slightly when given gas, however when you floor it, it doesn't seem to hesitate. It also doesn't seem to miss when reving the engine. I am assuming that the carbs just need a minor adjustment. However today while driving....aggresively and in extreme heat (it was over 33C with high humidity) I get the flashing (slow down) 5-8 cylinder light come on.

    I immediately stop.....but don't see any glowing cats, so I continue home in a much more leisurely pace. Slow down Lights do not come on again during the 1/2 hour drive home. (kept revs down, but speed up to keep the fresh air running through the car.

    I come home and attack the spark plugs in the back bank to see if there is an issue. 1st plug looks OK but has a little oil around the white part of the plug, the end looks OK as in no excessive deposits.

    So my question is......what's the problem....or do I even have one. Is it most likely just the carb kits that weren't adjusted properly....or do I have a back bank of plugs that aren't on at all times. Perhaps the wires are bad, or the extentions.

    Looking for any kind of feed-back before I head back to the dealership.
     
  2. thecarreaper

    thecarreaper F1 World Champ
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    Sep 30, 2003
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    my sympathies to ya!!!! my carb 308 has a backfire when shut off hot. most likely it seems from excess fuel getting into the header pipe and igniting. it seems to me if the carbs were rebuilt that they would have ops - checked them on a bench or something and offered to sync then on the car. i plan to syc and adjust mine asap, before the backfire damages something!! it would be wise to post the size of your idle and main jets to help determine the cause. mine are one step over stock so i have ruled out a lean condition until i can read the plugs. i am at sea level and am running 135 main jets and NGK BP5ES plugs, no cats or air pump, Ansa exhaust. i have dual dizzies with single points. ..... man i hate points .......... upgrades pending the $$$$$ ;) check Birdmans post on "CARB TUTORIAL" in the archives!!! cheers, michael
     
  3. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
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    JPL,
    Sounds amazingly similar to what mine was doing when it was running with idle jets that were too lean. Believe it or not, around town driving (under 4000 RPM) uses mostly just the idle jets for fuel (not the main jets) as they feed the progression hole circuit in the carbs as well as the idle. If the idle jets are too lean, the car will tend to miss a lot (popping). People tend to correct the idle mixture by turning the idle mixture screws all the way out, but that only sorta fixes it at idle, not "off idle" in the progression hole range (i.e. around town driving). This will also cause hesitation because when you press the gas, it leans it out even more before the fuel "catches up" with the vacuum. Generally the car idles like crap in this state, so the solution (bad solution) is that people turn up the idle speed. That causes it to backfire when you shut it off and pop a lot when idling.

    I would try one size richer jetting on the idle jets (not sure what stock was on the '78's but find out and go one size bigger). I went from .55 to .60 on mine and it was like a new car. VERY easy to do in a couple hours. Just pull the airbox, get a set of jets and put them in. Next, I would check the synch on the carbs, because that will cause hesitation too. This thread of mine has a lot of info on both of these procedures including sources of jets:

    http://ferrarichat.com/forum/showthread.php?t=20179

    Best,
    Birdman
     
  4. thecarreaper

    thecarreaper F1 World Champ
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    i need to find out what my idle jets are..... i bet if i go up a size it will help!!! ill ask Mike P when he has time if he can remember what size they are, it doesnt seem to be in the records, he did tell me the size of the mains though!!!! at least that nice 328 he has now wont have any of these issues!!!
     
  5. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
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    Michael,
    The idle jets on the left side of the left bank of carbs are accessible without removing the airbox. Just pull one and it says right on it what it is. It will take you longer to find your toolbox than to pull the idle jet to look.

    Birdman
     
  6. jpl

    jpl Formula Junior

    Dec 4, 2003
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    JP Lavigne
    Got at cylinder #7 spark plug and it is in terrible shape. Perhaps I have been running off of 7 cylinders which explains some of the hesitation. I will change plugs and move on from there. Thanks so far for the help.
     
  7. F308 MAN

    F308 MAN F1 Rookie

    Jan 19, 2004
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    Please keep us posted JP......as i am following this, and "Birdman's" thread with interest.
    cheers, david
     
  8. jpl

    jpl Formula Junior

    Dec 4, 2003
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    JP Lavigne
    Based on a number of notes within this and the archives I went off to do some spark plug shopping.

    I ended up with NGK BPR6ES plugs. Now what is the difference between BP6ES and BPR6ES (one is a resistor type and one isn't, but what does that mean exactly) and are both OK for my car.
     
  9. jpl

    jpl Formula Junior

    Dec 4, 2003
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    I did a little more checking on NGK's web site and found my own answer.
     
  10. thecarreaper

    thecarreaper F1 World Champ
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    thanks , been busy, will do!!!
     
  11. F308 MAN

    F308 MAN F1 Rookie

    Jan 19, 2004
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    An intimate and informal 308 carb "register" would be an idea, me thinks.
    To record and log owners plug type, jet sizes, altitude, oils etc. for reference purposes.
    I would find this a useful document at major service time.
    regards, david
     

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