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Slight backfiring when shut off

Discussion in 'Technical Q&A' started by yelcab, Jan 21, 2004.

  1. yelcab

    yelcab F1 World Champ
    Consultant

    Nov 29, 2001
    11,193
    San Carlos, CA
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    Mitchell Le
    My 75 308GT4 just recently started to give a very slight back-firing at shutting off, not like a pop, but more like poof. Carbs were all cleaned and balanced may be 2500 miles ago. Spark plugs changed 1000 miles ago. Temperature in Nor Cal is now colder, but only 50-60 ish.

    Is it something to look into? Or, may be just balance the carbs again this weekend?
     
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  3. Ferrari_tech

    Ferrari_tech Formula 3

    Jul 28, 2003
    1,527
    UK
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    Malcolm W
    Sounds like an ignition problem to me, are you sure that the ignition timing is okay, and when was the last time the ignition points checked, also check to see if the distributor advance isn't sticking or slow to return to normal.


    MW
     
  4. yelcab

    yelcab F1 World Champ
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    Nov 29, 2001
    11,193
    San Carlos, CA
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    Mitchell Le
    I am ... point-less now with the crane ignition systems in both distributors. Never go back to points again.

    I guess I should check the timing, the igntion system this weekend.
     
  5. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    2,559
    Chicago
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    Philip
    If the Crane system is like the Electromotive, it fires 2x on each (4-stroke) cycle. As a result, you can get a pop on shut down. Also, as the idle mixture gets rich, it'll do the same. Nothing to worry about in my experience.
    HTH
    Philip
     
  6. Mike328

    Mike328 F1 Rookie
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    Oct 19, 2002
    2,655
    Boulder, CO
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    Mike
    I get the same pop on shutdown since my last carb "maintenance." It didn't always used to happen.

    Stock ignition, but with an ANSA.

    Sometimes it can be loud. It's is tightly correlated with my current engine gas temperatures as measured in the rear header (I have a guage in the car.)

    I don't know WHY it does this; could definitely be a timing or ignition problem, though.

    I just had the carbs rebuilt and am doing a major next month; I expect it to go away then (especially with the new electromotive system). That said, as pointed out above the electromotive is a waste-spark system so pops on shutdown with that unit are not uncommon, I understand.

    --Mike
     
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  8. Mike328

    Mike328 F1 Rookie
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    Oct 19, 2002
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    Mike
    One trick to "fix the symptom" (not a solution!), as I learned reading FerrariChat a year ago or so, is to put the clutch in slightly (brake on!) to drop the RPMs to say 500-700, and then shutoff. That has always prevented a popping for me. Useful in parking lots or restaurants or other potentially embarassing areas ;).
     
  9. yelcab

    yelcab F1 World Champ
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    Nov 29, 2001
    11,193
    San Carlos, CA
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    Mitchell Le
    It is really not annoying at all, just a very small "poof." The ignition is the El Cheapo Crane XR-300 so I think there is only one spark per cycle per cylinder, not like the multi-thousand $$ Electromotive.

    Could be just a rich idle, it has been over 1000 miles since I messed with the idle screws.
     
  10. Ken

    Ken F1 World Champ

    Oct 19, 2001
    16,078
    Arlington Heights IL
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    Kenneth
    Typically this is from running lean. Check that first. When I had my carbs rebuilt I also acquired a backfire but a little tweaking eliminated it.

    Also, if you have a small dent in your headers near the engine, that can cause it too. (Carb guy not so carefull?)

    Tips until you fix it: let the car idle 20 seconds before you shut off. This cools the headers enough so it won't sneeze. Or as suggested, as you turn the key off, let the clutch out in gear (and be on the brake!) just before the engine stops. This will stop the backfire as well.

    Ken
     
  11. f355spider

    f355spider F1 World Champ
    Rossa Subscribed Owner

    May 29, 2001
    17,485
    USA
    My 78 308 GTS did this, but the backfire was very faint, and could barely be detected. I noticed it a bit more with removal of the cats, and richer jetting, but it was never a big deal or that noticeable to others.
     
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