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Exhaust pops

Discussion in 'Engine & Transmission' started by jae, Sep 15, 2007.

  1. jae

    jae New Member

    Messages:
    363
    Location:
    Ashgrove, Brisbane
    I have noticed over the last week that every now any then i hear a bit of a popping almost backfire sound coming from the exhaust. These are usually accompanied but kind of little flat spots, sort of as if the engine was running rough.

    From what i can tell it mostly happens when i'm cruising along and my foot is only just on the accelerator, or if i'm coasting down a hill in gear. Does anyone have an idea what could be causing this? Cheers

    J
     
  2. Organichead

    Organichead New Member

    Messages:
    232
    Location:
    Adelaide Hills, SA
    Hi Jae,

    My beetle was doing the same thing mainly when coasting down hill. It was due for a tune and service and alls well, so I reckon that timing may be the thing. Hope this helps................org.................
     
  3. kinga

    kinga Member

    Messages:
    146
    Location:
    Perth WA
    Vacuum line leaks often cause the pops because it mucks up the timing (vacuum advance), a vacuum leak would also cause a flat spot too.
     
  4. cbus

    cbus Well-Known Member

    Messages:
    13,562
    Location:
    sunshine coast
    Do you have extractors or hot dog exaust system?

    Could be because of reduced backpressure or lean fuel mixture.

    col
     
  5. jae

    jae New Member

    Messages:
    363
    Location:
    Ashgrove, Brisbane
    Nope all standard exhaust back there. Whats the best way to check for vacuum leaks? Its seemed to be running well the last few days then tonight it started up again. Although it could be just that i notice it more at night.
     
  6. KahunaKombi

    KahunaKombi Super Moderator Staff Member

    Messages:
    22,281
    Location:
    Bracken Ridge, Qld
    What may seem as an exhaust pop could actually be a pop back through the carbies
     
  7. jae

    jae New Member

    Messages:
    363
    Location:
    Ashgrove, Brisbane
    I've heard carby pops before and the seem like more of an internal noise. The peticular brand of pop i have, i usually need the window open to be able to hear them. But thanks anyway Bert.
     
  8. KahunaKombi

    KahunaKombi Super Moderator Staff Member

    Messages:
    22,281
    Location:
    Bracken Ridge, Qld
    No Probs Jae - may be an exhaust gasket going and when the hole gets big wnough you'll get a backfire as happened on Boris the Type3 Squareback I owned - threw flames as it had a hot dog muffler :) was excellent
     
  9. Blue76Bay

    Blue76Bay New Member

    Messages:
    26
    Location:
    Western Sydney NSW
    If you have a Type IV motor, remove the tin covering the pushrod tubes and then check to the tightness of the exhaust manifold nuts. I recently came back from Queensland and everytime I started the car I would get some back firing when idling then after a while it dissappeared. I thought I needed to tune the car or I had a vacuum leak also. It turned out to be the exhaust manifold nuts had worked loose. Actually one nut had come off completely. I found the nut sitting on the tin I removed. I put it back on and tightened everything up. There was enough room on the threads to put a second nut to lock it down and hopefully stop it working loose again. Exhaust manifold studs corrode over time and with heat also seem to loose some of the thickness on the thread.
     
  10. kombidaze

    kombidaze Member

    Messages:
    466
    Location:
    Uki NSW
    this is some info from a carby guru this is his web site if you are interested relates mainly to webers but can be applied in general to all carby situations
    http://www.carburetorclinic.com/tech_articles.htm
    here is the info
    First you need to know WHY this happens. Exhaust backfires are from unburnt fuel that ignites in there, making it like a cannon. There are different reasons why this can be the case, so you need to make sure the things NOT related to the carbs are not causing it. Those causes are ignition misses and exhaust leaks. Ignition misses cause the fuel charge to get forced out to the exhaust without actually igniting. That added fuel in the exhaust WILL ignite in there, since it mixes with a lot of hot gas, goes through the exhaust transformations and eventually pops. Exhaust leaks facilitate this by sucking raw air in between exhaust pulses, but it's not necessary in order for it to happen. As soon as the raw fuel meets any raw air, it's going to ignite.

    Now, related to the carbs, a rich condition will cause it. If your ignition is working perfectly, and you have a rich condition and big exhaust, it will happen. It can also happen with a smaller exhaust system, but it takes a little more richness to make it likely. Something is too rich. Even if your jets are right, when you decelerate you might just be getting a rich condition from your mixture screws.

    Keep in mind that in a highly tuned VW, some popping on deceleration is just likely to happen. When the throttle valve is in the idle position, fuel does not flow out of the main circuit, just the idle circuit. The combined effect of the closed throttle and elevated engine rpm is to create a high vacuum in the intake manifold. This vacuum causes a high air flow rate through the small gap formed by the throttle plate and carburetor throat. and the idle circuit will not deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there. When the fuel in the combustion chamber DOES fire, it's not fast, and the combustion still occurs as it is shoved out the pipe, where it sometimes ignites the raw mixture that has collected and the exhaust then pops or backfires. Mine does it, and it's normal to a degree, as long as it's not shotgun type fire that sends everyone around you hitting the deck and yelling "GET LOW! GET LOW!". Those types of backfires are accumulations after the collector that are caused by rich conditions and ignition misses, as well as leaky exhausts.
     
  11. KahunaKombi

    KahunaKombi Super Moderator Staff Member

    Messages:
    22,281
    Location:
    Bracken Ridge, Qld
    Top post Kombidaze :)
     
  12. Schmoburger

    Schmoburger Active Member

    Messages:
    4,382
    Location:
    Nowra/Jervis Bay area, NSW.
    Also, I found that with poorly synchronised carburetters, I also got popping from the exhaust... most noticable on deceleration. tuning the carbs properly fixed it and made it run a lot better. :)
     

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