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LOUD POP POST SHUTDOWN FROM EXHAUST

Fuel and Air Systems

  1. Cory

    Cory Member

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    I have a 73 Challenger 440RB with Edelbrock AVS. The engine is a stock 72 HP2. My timing is set at 16* initial and 32* total all in around 2800 rpms. With vacuum advance hooked up, I’m around 50 to 55*. I’m running 93 octane gas. I’m idling at 880 to 900 rpm in gear warmed up and 1050 to 1100 rpms in park / neutral. When I shut off the car, I get a loud single pop from the exhaust. Any ideas?
     
  2. Adam

    Adam Well-Known Member

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    From what I have read, I believe you may have an exhaust leak which allows air to get into the exhaust system; combined with an over rich idle, resulting in the gas igniting. Try to lean up the idle circuit, and look for exhaust leaks at the heads and collectors/head pipe.
     
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  3. Cory

    Cory Member

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    I leaned out the idle mixture. I was a turn and a quarter out on each air / mixture screw. I turned them in a quarter turn. I’m now one turn out on each screw. I corrected an exhaust leak the other day on the passenger side at the head pipe. I got the car warmed up this morning and then shut it off. Still a single pop about a second after I shut it off. I’m about to drive it to work. It’s a 20 minute drive. I’m curious to know now if I need to make an adjustment on the electric choke now. I’ll update.
     
  4. DetMatt1

    DetMatt1 Well-Known Member

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    Does it happen often?
     
  5. Rich G.

    Rich G. Well-Known Member

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    Why do you have the idle so high? You’re probably still dumping fuel in the engine at that high and idle and that’s the backfire you’re getting.
     
  6. Cojohnso

    Cojohnso Active Member

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    What is the age of the cam? And it's profile? Is this issue new? And getting worse?
     
  7. Cory

    Cory Member

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    It seems like the car just doesn’t idle right at 800. I’ll mess around with it and see if I can find a happy medium between 800 and 900. I let you know.
     
  8. Cory

    Cory Member

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    Every time.
     
  9. Cory

    Cory Member

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    Not sure how old the cam is. Profile? If my initial timing is around 20*, it doesn’t do it. 18* it only does it on a hot day.
     
  10. AUSTA

    AUSTA Well-Known Member

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    Does it ping under acceleration when hot
     
  11. Cory

    Cory Member

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    What do you mean by ping? It accelerates kind of slow at first and then it goes. Also I can stomp the pedal or it will bog down. If I push the pedal 3/4 of the way, it accelerates rather quickly. It does make any popping noises or backfiring.
     
  12. AUSTA

    AUSTA Well-Known Member

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    Like a metallic ting ,ting under heavy acceleration
    Is that does or doesn't make any popping or backfiring what temp does the engine get to when hot
     
  13. Cory

    Cory Member

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    I haven’t heard anything like that. When it’s warm, it’s around 180* but slowly continues to climb to around 196. On a hot day, it will continue to climb around 206. Once I start driving it drops down to 180 and holds there.
     
  14. AUSTA

    AUSTA Well-Known Member

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    Does the engine try to run on when the key is turned off when hot
     
  15. Cojohnso

    Cojohnso Active Member

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    Cam lift, duration. Hydrolic?
     
  16. Cojohnso

    Cojohnso Active Member

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  17. Cojohnso

    Cojohnso Active Member

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    That is sounding more like a small vacuum leak? Secondaries stumbling at correct timing? If you advance? It goes away. This of course is "Off the cuff" diagnosis. Spray around intake and carb mount with carb cleaner to see if there is an up tick in idle?
     
    Last edited: Mar 13, 2019
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  18. AUSTA

    AUSTA Well-Known Member

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    Also check to see if you have fuel running into the manifold when after the key is turned off
     
  19. Rich G.

    Rich G. Well-Known Member

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    If that’s a stock 440 and you can’t idle it down and it bogs and hesitates it sure sounds like you have a serious carb problem.
     
  20. DetMatt1

    DetMatt1 Well-Known Member

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    You’re saying it’s coming out the exhaust this pop, is it coming from the engine or is it happening in say the muffler?