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New Motor Backfiring badly, Please help

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Waxer, Sep 23, 2001.

  1. Waxer

    Waxer 1/2 ton status Author

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    OK. Just slapped in a rebuilt 350 from Kragen Auto parts. Goit a good deal on it. I also just put a new carb on it. Remanufactured one from Autozone. It idles beautifully.. nice and smooth and idles at 700rpm very nicely. Accelerates good as well. When I'm idling and I rev up the engine, it backfires as the rpms come down. Does the same thing when I am driving and accelerate pretty hard and let off the gas to shift into 2nd or 3rd. Does this sound like timing to you? I dont have a timing light and my harmonic balancer is off so a timing light wont work here. Would you recommend I advance it or retard it? Thanks in advance...

    ps. what causes backfiring? What exactly is backfiring?

    Crawlin the rocks with my K5
    <a target="_blank" href=http://www.rockreadyk5.com>http://www.rockreadyk5.com</a>
     
  2. jimmyjack

    jimmyjack 1/2 ton status

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    When I rebuilt my engine yrs ago I had a problem with the same thing. Not really a backfire but more of a rumble from the exhaust. Everything was fine, timing, plugs. I let it slide for a few yrs and then a mechanic told that my anti back fire valve wasn't hooked up! It's near the smog pump. Hooked it up and it all went away! I had no idea what I was doing when I hooked the vacuum tubes back up. I've since torn most of it off (under mechanic supervision!)

    That jeep thing? Yah I understand it....... Like my Blazer? <a target="_blank" href=http://community.webshots.com/user/blazerk5>http://community.webshots.com/user/blazerk5</a>
    Jim
     
  3. solowookie

    solowookie 1/2 ton status

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    you sure you got this timed properly???

    I just did my first rebuild, and I'm still fairly green at this stuf... I've had some similair issures that were all timing related...

    <font color=blue> Jeff - may the force be with you</font color=blue>
    <a target="_blank" href=http://coloradok5.com/gallery/Jeffs-Stuff>link to k5's</a>
     
  4. 74Roadster

    74Roadster Registered Member

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    Most likely your idle mixture is set way too rich, the rumbling backfiring sound you're getting is the unburned fuel burning in your exhaust. My Jimmy did this for a long time and i hated it, then i had a freind that's real good with carbs show me how to set the mixture properly and not only did it run better but the backfiring in my exhaust went away.



    74 roadster in progress [​IMG]
     
  5. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    I bet you anything the valve lash needs to be set. Had the same problem on my trial run after rebuilding the top half of my dads Jimmy. All you have to do is take off one valve cover at a time. Get a 3/8" drive ratchet and do each rocker with the motor running, make sure its warmed up. You loosen the nut on it till you start to hear the rocker ticking. Then you tighten the nut every so slowly till you hear the ticking quit. Then you tighten it one half turn past when you heard the ticking stop. Stop the motor, put on the cover and do the other side.

    &lt;a target="_blank" href=http://www.geocities.com/bigkern76&gt;http://www.geocities.com/bigkern76&lt;/a&gt;
     
  6. 78Blazer

    78Blazer 1/2 ton status

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    Id have to say that it is probably the air/fuel mixture on your carb. I just rebuilt my carb and it does the same thing when I jump on it and let off.. I just havent bothered to readjust it yet.



    Scott
    78blazer
     
  7. 4x4k20

    4x4k20 1/2 ton status

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    you need to check the little things first such as the firing order and check the valve lash i think you have a crossed wire it easy to cross them

    4x4k20
     
  8. Waxer

    Waxer 1/2 ton status Author

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    Thank you for the many responses and ideas.
    I know for a fact the firing order is good so that rules that out. I have no adjusted the valve lash since first running this motor. I bought it as a long block so that is one possibility. Today I advanced it as far as the firewall would let me and it backfired less but when I tried to start it I got predetonation so I'm as far as I want to mess with the timing. I also have not adjusted the carb since buying it. It runs so smooth I dont want to touch it :) But I do smell a little bit rich of an exhaust. So I'll look at adjusting the carb first and see how goes. If it still does it then I'll do the valve lash. Thank you all for the help.

    Crawlin the rocks with my K5
    <a target="_blank" href=http://www.rockreadyk5.com>http://www.rockreadyk5.com</a>
     
  9. muddin4fun

    muddin4fun 3/4 ton status

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    Is it backfiring thru the carb or thru the tail pipes?
    Pull the plugs and see if they are reading rich. Also, check the exhaust system for holes. You need to check the timing on it also. What kinda mufflers are you running?

    Backfires come from unburned fuel getting hot enough to explode (running too rich or not timed properly), exhaust leaks will cause it too.....




    If you see my truck on a trailer, call 911 cause it's <font color=red>STOLEN!!!</font color=red>
    Website link is in my profile [​IMG]
     
  10. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    That also reminds me to check if you're distributor is on no.1 or not. Take off the number 1 plug and stick your finger in the hole and bump the motor. You should get a woosh of air when you're moving no.1 to TDC. Then what you'd normally do is move your timing mark to 0 degrees but since yours is off you'll have to feel where the piston is at with a piece of wiring of something. You can move the motor a little bit by using a wrench on the front of it. Then pull of you're distributor cap noting where the no.1 plug wire is and the rotor should be pointing to it. That'll defintely screw things up if its off. If it is then put down a post and we'll show you how to drop the distributor in right. If you already know how to do this so i wasted my time, but i always gotta throw in my $.02.

    <a target="_blank" href=http://www.geocities.com/bigkern76>http://www.geocities.com/bigkern76</a>
     
  11. jimmyjack

    jimmyjack 1/2 ton status

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    Yes, you can be off 180 degrees on the timing and the engine will run pretty well but when you load it up it'll backfire.

    That jeep thing? Yah I understand it....... Like my Blazer? <a target="_blank" href=http://community.webshots.com/user/blazerk5>http://community.webshots.com/user/blazerk5</a>
    Jim
     
  12. RedDwarf

    RedDwarf 1/2 ton status

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    Bunk. Engine will not run 180 deg out. Think about it. You're getting spark when the exhaust valve has just closed after the piston pushed out the fuel mixture. There's nothing in there to burn.

    Valve lash, valve/seat seal, can cause it. It just redid an engine that was backfiring because one of the cam lobes had completely worn off and the exhaust valve for that lobe was never opening. Did you break in the cam when you started the engine? Maybe they didn't replace the cam at all?

    Poundin' the ol "pavment"!
     
  13. 74Roadster

    74Roadster Registered Member

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    I've never had a motor 180 degrees out even run, let alone somewhat smoothly...



    74 roadster in progress [​IMG]
     
  14. Waxer

    Waxer 1/2 ton status Author

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    It is backfiring through the tailpipes. I can hear it coming from there. I also have some exhaust leaks. I welded the pipes back up myself but couldnt get the top 25% of the pipe that well cause it was too close to the body and my MIG gun wouldnt fit. So yes it is leaking in that area. The vacuum line to the valve on the smog pump isn't hooked up either. Should that go to a constant vacuum source?

    Thanks again for all the great ideas..

    Crawlin the rocks with my K5
    <a target="_blank" href=http://www.rockreadyk5.com>http://www.rockreadyk5.com</a>
     
  15. dyeager535

    dyeager535 1 ton status Premium Member

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    I'd first fix the exhaust problem.

    Couple of motors I've had an exhaust leak on, backfiring on deceleration (idle or not) was the norm. Fixed exhaust, enver heard it again.



    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</a>
    No anti-theft measures on your truck? No pity when its stolen
     
  16. 84_Chevy_K10

    84_Chevy_K10 Banned

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    If it's not a normal sound, which I haven't heard it, I don't know, but I suspect that the choke is sticking or set too lean.

    Does it sound like the crackling sound from a straight pipe, or is it backfiring?

    Backfiring can be traced to carb, ignition, or valves every time.

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     
  17. BorregoK5

    BorregoK5 1/2 ton status

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    I've had old honda and toyota 4 cyl motors run 180 degrees out, wouldn't idle but would run and backfire at part throttle and stumble up a thousand rpms or so at full throttle. I've heard of chevy small blocks doing that as well but never witnessed one.

    What, Me Worry?
    <a target="_blank" href=http://www.echobit.com:81/k5/> The 'BEFORE' pictures</a>
     
  18. muddin4fun

    muddin4fun 3/4 ton status

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    I put a cam in my pickup and had it off 180. It idled like crap, drove like crap, pissed and moaned about everything....
    It wasn't running 'efficiantly' to say the least. [​IMG]



    If you see my truck on a trailer, call 911 cause it's <font color=red>STOLEN!!!</font color=red>
    Website link is in my profile [​IMG]
     
  19. RedDwarf

    RedDwarf 1/2 ton status

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    Not to mention there's no fuel in the cylinder when the spark is delivered, and if there were, it would have to exhaust through the intake valve and vice versa.



    Poundin' the ol "pavment"!
     

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