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NEED HELP!!! exhaust backfires.....

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by StageZ, Apr 28, 2002.

  1. StageZ

    StageZ Registered Member

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    I just set a new cam... and I can't get her to stop backfiring.... I set the timing and everything seems to be fine, then when I start her the next time... she backfires RIGHT AWAY(around 400+ RPMs.... I have no exhaust leaks and no manifold leaks... I have no other ideas... spark seems fine(new wires,MSD dirstributer, MSD 6a ignition)... need help... Could it be my choke? as soon as I advance the timing she seems ok... then I fire her up and again with the backfires.... any ideas will help... thanks in advance....
     
  2. 4x4k20

    4x4k20 1/2 ton status

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    re check you firing order sounds like you got a wire crossed if its a chevy v8 firing order is 18436572 clockwise rotation also recheck the rocker arm adjustment to make sure you dont have one to tight
     
  3. StageZ

    StageZ Registered Member

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    I cut and marked the wires myself (they are in order) and re-ajusted the rocker arms back to a 1.5 ratio... I was running a 1.6 ratio and thought the valves were staying open too long... still no luck...
     
  4. Jay73K20

    Jay73K20 Registered Member

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    "I set raw timing" Raw=initial? What did you set it at? I didn’t know there was a difference between 1.5/1.6 rocker arm valve lash setting. I always thought the difference was between the lifters; stock hydraulic, high pump hydraulic and solid for a SBC. Why are you idling so low? I can see 500 maybe but not for my truck, 700-800 is where I like it. If you have a 1:3 ratio for your alternator pulleys then you will have about 2250-2400 for your alternator shaft which is right where it needs to be for decent charging, not max charging, that is more like 6000. When you advance the timing what is you rpm? Can you idle at 400 anymore? Are you sure you got the cam in properly. I've seen a couple of SBC where they took the keyway on the crank and lined it up to the cam sprocket dot like a Ford. That wasn't to fun to trouble shoot the first time.
     
  5. 4x4k20

    4x4k20 1/2 ton status

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    i would go back and check the firing order to see if you have a plugwire crossed i have done it several times myself usualy number 5 and 7 at least you should check it if thats not it how are you adjusting your valves ? and did you line the timing marks on the cam gear and crank gear?
     
  6. StageZ

    StageZ Registered Member

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    I must have explained something wrong.... when I said 400RPMs, I didn;t mean that was where I was idling... I meant when I started her, she backfired as low as 400 RPMs(on the way up to idle)... I set her Idle at 1200.... I have a 180A alternator and charging is not the problem... the cam is set up right... I measued the lift and duration myself....
     
  7. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    <blockquote><font class="small">In reply to:</font><hr>

    and re-ajusted the rocker arms back to a 1.5 ratio... I was running a 1.6 ratio and thought the valves were staying open too long... still no luck...

    <hr></blockquote>
    There is no way to adjust your rockers to one ratio or another. The physical length from the fulcrum to the rocker tip determines rocker ratio, it's not adjustable. How exactly are you adjusting your valves?

    Rene
     
  8. StageZ

    StageZ Registered Member

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  9. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Well that bit of info wasn't posted earlier...

    Rene
     
  10. StageZ

    StageZ Registered Member

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    Did I mention I love toys?
     
  11. reddog64

    reddog64 1/2 ton status

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    did you tiome it with a light? it's possible your one tooth off on the distributor...
     
  12. StageZ

    StageZ Registered Member

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    I timed her by light, by ear and by vacuum.... still no luck.... I can get her to run fine at 4 BTDC but, when I shut her off and start her again she runs like [censored] and backfires again... I can't figure it out....
     
  13. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    Time to pull the timing chain cover off!
     

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