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Intake backfire cause????

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by colorado_emt, Jan 8, 2002.

  1. colorado_emt

    colorado_emt Registered Member

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    My '79 350 has an intake backfire at cruising speeds. It just happens without any change in throttle position. it also does it if i punch the throttle from low speeds. I have heard several possibilties but would like your opinion on what it may be so i can explore that path. Thanks.

    Chris
    <font color=red>79 K-5</font color=red>
     
  2. HarryH3

    HarryH3 1 ton status Author

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    Timing way off? A couple of spark plugs wires going to the wrong plugs? Crossfire causing the wrong plug to fire at the wrong time? Intake valve sticking open?

    Those are the things that come to mind. To backfire through the intake, the intake has to be getting exposed to the fire in the combustion chamber somehow.

    <font color=black>HarryH3 - '75 K5</font color=black>
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  3. Burbinator

    Burbinator 1/2 ton status

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    Might be a vacuum advance problem. Leaky diaphram, stuck linkage, etc.

    Haze gray and underway...<font color=blue>or blue, in my case.</font color=blue>
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  4. dogman

    dogman 1/2 ton status

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    I would check for:
    a flat cam lobe ? poping
    funky hei chip? runs like $hit
    timing+wires in fire order? &lt;someone messin with it.
    good luck.
     
  5. colorado_emt

    colorado_emt Registered Member

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    I've thought about timing and plug wires but the truck runs fine. and then it backfires. it's very random!

    Chris
    <font color=red>79 K-5</font color=red>
     
  6. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Sticking valve maybe. That can be random. I would do a compression check on it and see what you come up with. Also do a poor mans blled down by taking the srader valave out of the hose on the compression checker and put some air to the cylinders. You hear air in the oil fill you got ring blow by...some is normal on a high mile engine. hear air in the intake you got intake valve sealing problems. Exhuast same deal.

    Women dig dents and flat paint!
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  7. dyeager535

    dyeager535 1 ton status Premium Member

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    I just got done fixing a "backfire through the carb from a stop" problem on my truck that existed since I threw the truck together in a hurry. Finally wouldn't start one morning (of course fired up when I got back home) but it turned out everything that could be wrong with the distributor, was, when I figured I'd swap ignition modules. Corroded cap/rotor contacts, loose rotor, and the distributor weights were stuck from bad bushings. Once I fixed all of the above, and re-adjusted the timing, I have no problem with backfire or sudden loss of power when I floor it. Just an experience...

    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>www.dorianyeager.com/index2.html</a>
    Why insist on counting when the ring gear has the tooth counts stamped in?
     
  8. Slayer

    Slayer 1/2 ton status

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    Recently fixed a problem like that. Mine was cold starts in the morning, especially the chilly eastern Colorado plains mornings. Then it would run fine, and while cruising at a constant speed, it would cough and belch through the intake. Scorched a paper air filter (I usually remove the K&amp;N in winter as it retains too much moisture, which then freezes).

    Turned out to be the manual choke cable had slipped and was way out of adjustment. Several of the people here at CK5 told me about it possibly being a choke problem, and that's what it turned out to be.

    Ol'Blue's running fine now, new choke cable, adjusted the actuater bracket (you gotta smack it with the sledge hammer JUST right), and all is good.

    Now if I could only get a lift-kit for my garage[​IMG]...[​IMG]...[​IMG]...[​IMG]...[​IMG]...[​IMG]...[​IMG]...[​IMG]...




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  9. colorado_emt

    colorado_emt Registered Member

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    my problem also seems to effected by the cold it seems i had no problem during the summer but since it has been cold it has gotten worse. this sounds like my exact symptoms . I will check my choke tomorrow. Thanks for all the advice! now I can start to rule out possible problems

    Chris
    <font color=red>79 K-5</font color=red>
     
  10. high&liftedup

    high&liftedup Registered Member

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    I also recently had this problem.... after a lot of trial &amp; error &amp; new parts, I found the ACCEL distributor shaft was loose, thus causing the rotor to move up against the cap.... the final result was a bent valve....ITS definitly a timing thing!!!!

    HE will always get you out of the mire clay!!!
     
  11. Slayer

    Slayer 1/2 ton status

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    <font color=green>Just ran across a buddy who had a very similar problem, except that the popping and backfires were happening at no specific time. Sometimes while idling, sometimes under load, sometimes while cruising, etc.

    Turned out to be 2 bad spark plugs. This was after tearing the carb apart, rebuilding the distributor &amp; coil, compression checking the cylinders, triple checking the battery, triple checking vacuum, and running the wiring harness for possible shorts. Only 1 plug looked bad, but replacing them solved the problem. Had him stumped bad enough that he was actually no longer enjoying working on his truck.
    </font color=green>

    <font color=blue>I don't know everything, I just think I do.............</font color=blue>[​IMG]
     

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