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back firing through the carb

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by xnoahx, Jun 5, 2001.

  1. xnoahx

    xnoahx 1/2 ton status

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    OK, i just got my vortec heads on and it starts up and it will idle just fine but when i give it gas it hesitates and backfires through the carb. What could cause this, maybe the intake valves arent closing all the way? could it be the carb is too lean? any help would be much appreciated.

    Check out my 73 K5 http://members.theglobe.com/xnoahx/73k5.htm
     
  2. ericsoffroad

    ericsoffroad Registered Member

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    i would run a compression check to see if your valvs are closing all the way if the valvs are doing there job check timing the timing may be to far advanced
    just my 2 cents


    are we having fun yet
     
  3. Blazer1970

    Blazer1970 1/2 ton status

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    Check at least two more times to make sure that you don't have two spark plug wires crossed. A V-8 will idle surprisingly well on 6 cylinders.

    Tim

    70 Blazer CST 4X4 350 SM465 NP205
    87 Burb 4X4 350
    01 GMC 2500HD 4X4 Duramax/Allison
     
  4. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Did you do any work on the cam i.e. replace it or the chain? When you install a cam the 2 dots facing...this is not TDC. If the dots were not pointing up on both cam and crank you installed the distributor 180deg out. Pull up on the distributor but don't turn it tun the crank one full turn and drop the distributor back in.

    Diging it in the dirt with my K5's
    Grim-Reaper
    <A target="_blank" HREF=http://grimsk5s.coloradok5.com/>http://grimsk5s.coloradok5.com/</A>
     
  5. xnoahx

    xnoahx 1/2 ton status

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  6. HarryH3

    HarryH3 1 ton status Author

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    70 psi is way too low. 150 is more like what I'd expect to see, depending on your altitude. It drops slightly as altitude increases.

    <font color=black>HarryH3 - '75 K5</font color=black>
    <A target="_blank" HREF=http://ThunderTruck.ColoradoK5.com>http://ThunderTruck.ColoradoK5.com</A>
    It's a great day to be alive...
     
  7. xnoahx

    xnoahx 1/2 ton status

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  8. yunit

    yunit 1/2 ton status

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    Try adjusting the valves they are probably to tight, if you are getting 70psi. You could be running a little lean, does your exhaust have a funny smell now? Remember your flowing a little more air now, so will need to flow a little more gas.

    1978 K5, Palisades Park, NJ 07650
     
  9. Storm Trooper

    Storm Trooper 1/2 ton status GMOTM Winner

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    1st, did you find anything?
    2nd, lets start all over, what motor are you running?
    You said you put new heads on, did you change manifold, dist., carb., rockers, lifters, push rods, cam etc.? Did you turn the crank at all while apart? Did you put the distributer back in the same direction if not? Proper imformation from you will make it much easier to help you out.

    [​IMG]<font color=red>[/b]"Rid'in HigH in my K5"[​IMG]
    ....[​IMG]<font color=blue>[/b]"6" Lift -<font color=purple>38.5's"[​IMG]....
     
  10. Blazer1970

    Blazer1970 1/2 ton status

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    I get 180 - 190 psi compression with my motor with Vortec heads and stock type dished pistons.

    Tim

    70 Blazer CST 4X4 350 SM465 NP205
    87 Burb 4X4 350
    01 GMC 2500HD 4X4 Duramax/Allison
     
  11. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    70! wooooo thats big time low. Uhhhhhhhhhhhhhhh back to my question...did you mess with the cam? Now it's sounding the the cam is not indexed or the valves are not closing or you wasted a bunch of time with a motor with bad rings but even bad rings should pull over 100.

    Diging it in the dirt with my K5's
    Grim-Reaper
    <A target="_blank" HREF=http://grimsk5s.coloradok5.com/>http://grimsk5s.coloradok5.com/</A>
     
  12. HarryH3

    HarryH3 1 ton status Author

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    Here's another thought, did you adjust the rockers properly? If the adjusting nuts are too tight then the valves won't close quite all the way.

    <font color=black>HarryH3 - '75 K5</font color=black>
    <A target="_blank" HREF=http://ThunderTruck.ColoradoK5.com>http://ThunderTruck.ColoradoK5.com</A>
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  13. xnoahx

    xnoahx 1/2 ton status

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    its a 350 i did a cam change a while back and it has ran fine so thats not the problem, when i did the heads, i also switched to a edelbrock performer for the vortec heads and put in an HEI. The distributor is in right because i know what cylinder its on when i adjust the valves. Im going to try loosening all the valves a bit. It mights just be my compression tester is bad, ill try to get another reading pretty soon today, im just letting the engine cool a little before i touch it. im also going to play with my vacuum advance a little, its an adjustable unit on the distributor so maybe its set too far advanced.

    Check out my 73 K5 http://members.theglobe.com/xnoahx/73k5.htm
     
  14. xnoahx

    xnoahx 1/2 ton status

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    Ok, the tester is junk, its letting some of the pressure escape through the release valve so i have more than 75 psi per cylinder i still need to try the valves

    Check out my 73 K5 http://members.theglobe.com/xnoahx/73k5.htm
     
  15. dyeager535

    dyeager535 1 ton status Premium Member

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    Well if everything is setup correctly, vacuum advance won't have any effect (ok realistically there is a bit of "leakage" so there might be a tiny bit of advance) on idle. Should be run on "ported" vacuum only, no vacuum to advance cannister at idle.



    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. CV202

    CV202 1/2 ton status

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    Had this happen on an old Buick I had. Check to make sure the EGR is working correctly. For some reason it started puking flames at me when the EGR crapped out. Wierd eh?

    Why do people keep calling it a Bronco!!!!! [​IMG]
     
  17. 84_Chevy_K10

    84_Chevy_K10 Banned

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    Just lose the EGR completely.

    I'm suspecting valvetrain. Could also be timing or carb related, but I say valvetrain first.

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

    xnoahx 1/2 ton status

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