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  1. #1
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    Default acceleration backfire

    So here is my problem and i apologize for the long post. i just want to get all the information that may be needed for diagnosis out there. The car just started backfiring the other day. Up until a week ago it ran great. I took it out and it started. Light acelleration and it runs but there is a little bit of stumbling.

    heavy throttle from any starting point to any finishing point and it backfires.

    at first i thought pump cam but if that were the case it would be doing it all the time and not just all of a sudden start.


    here is the setup

    72 454/th400
    holley 750 vacuum secondary
    primary jet 74
    secondary jet 80
    secondary spring blackm (tried yellow and silver as well)
    pump cam orange (both holes tried)
    the holley throttle body with a choke tower was replaced by a proform center with a milled top for air cleaner clearance
    the timing check revealed 10 degrees at idle and 36 degrees at 3000
    new plugs, wires, cap, condensor, points, rotor,

    it backfires regardless if the vacuum advance is hooked up to manifold, ported or plugged entirely.

    i have went through 2 cans of carb cleaner looking for a vacuum leak but cant find one.

    at idle in park a vacuum gauge says right at 18
    idle quality is smooth. when i watch the timing mark at idle with the light it doesnt move or jump idling

    in park if hit the pedal there is no hesitation or backfiring. as soon as i touch the lever the pump shooter squirts gas and it goes to WOT like it should. the backfire only ocurrs under load. this led leading me to believe there is something up with the secondaries so i wired the secondaries shut and tried again. still goes bang.

    i am tempted to pull the carb off of my old mustang and try it for comparison because that one runs perfectly but since it does run great i am hesitant to mess with it.


    i guess i could go back to basics and try a new set of wires just for giggles. since it was running good and then just started backfiring a burned wire seems like a logical explanation but why would in park all the way to wot run smoothly but under load open up WW3.

  2. #2
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    Check your timing.if you are getting several rapid backfires check for a bad exhaust lobe on cam.

  3. #3
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    also check for bent or broken pushrod

  4. #4
    Member Ezmunie's Avatar
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    I had the same problem once and it turned out to be the wires. Sometimes the new wires aren't what they are supposed to be, or there is one not on the plug well, but I figure you have already checked that.

  5. #5
    Member 73shark's Avatar
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    Could also be a bad distributor cap or less likely a broken valve spring.

    You didn't say but is it backfiring thru the exhaust or intake?
    Save the wave

  6. #6
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    it is going through the intake. i dont think its the cam lobe. i pulled the covers and all of the rockers are moving the same amount.

    i am thinking plug wires too because it just started all of a sudden.2 weeks ago it was running great.

    however, since it idles smooth, and in park will go to WOT with no backfiring or hesitation would a plug wire still be a culprit.
    i have had burned plug wires before and it would idle like crap.

    i have had vacuum leaks or timing issues that would cause it to backfire in park.

    i have never seen one backfire only under load but other wise run great.
    even gently easing on the gas in gear and there is no backfire.

    only when i hit the gas hard.

    makes me think pump shooter or pump cam

  7. #7
    Member Wayne robson's Avatar
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    I guess you are still useing points if so check the dwell,and your point gap,this can happen if the gap closses up, because of worn carbon vibre lobe in the distributor, and dont rely fully on the dwell as if points are burnt badly can do a similiar thing,and cause an incorrect dwell reading,this happened to me years ago on a 1972 porsche 911,it only had 3 to 4 thou point gap and it caused it to back fire like your vette. regards.

  8. #8
    Member gmjunkie's Avatar
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    I'd try re-adjusting the lifters!!My LS6 Big Block in my car hauler did this same thing several times till I reworked the heads and up-graded to Harder Valve Seats and Manley Swirl Polished Stainless Valves!!It was a case of receding valve seats from running Un-Leaded fuel in a older engine!!
    It's got so it seems to me that the Internet has created a lot more Horses A$$'s than we have Horses!
    junk!~!!
    Founding Member: 10 Corvettes Anonymous


  9. #9
    acceleration backfire Curtis's Avatar
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    My 2 cents puts the problem in some part of the ignition.. Cracked spark plug, arcing wires or cracked cap.. These are the only things can can go wrong in an instant. All the other stuff seems to me you would have seen some sort of progression to the point you are now.

  10. #10
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    i am going to totally eliminate the ignition from the equation by totally replacing it.

    i am going HEI, new wires, and plugs

    is it possible that the engine will run smoothly at idle and then at higher rpms stop

    its just kinda funny that the backfire happens right at 2500 rpms pretty much right at where the mechanical advance is supposed to be all in.

  11. #11
    Member 73shark's Avatar
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    Quote Originally Posted by baxsom View Post
    is it possible that the engine will run smoothly at idle and then at higher rpms stop
    Yes. What's happening as you increase the load on the engine, the plugs become harder to fire and the voltage will find the easiest path to ground or another plug that's not under compression. That's why I suspected the cap or wires. It sounds like you are getting crossfire and one or more plugs are firing before the intake valve is completely closed.
    Save the wave

  12. #12
    Technical Writer for Internet & Print Media
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    I've been reading this one with interest. It will be interesting to read what "baxsom" reports after going through the ign. system.

  13. #13
    Registered User wallyknoch's Avatar
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    U may have a broken valve spring. It obviously starts and runs good at idle. Not the ignition or carb as too many parts have been changed to eliminate ignition and carb issues. Check the valve springs. The answer may be there.

  14. #14
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    if its a valve spring i cant find it.

    i took the covers off and watched. no spring looked like it was broken. i checked each one as much as i could without removing them and nothing looks like it is a problem.

    what exactly should i be looking for, would it be obvious

  15. #15
    Member 73shark's Avatar
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    I've broken two valve springs and they were both obvious. Later found out that even tho it was a L-82, the valve springs were just standard small block springs. Replaced w/ Chevy hi-po springs and end of problem.

    They seemed to break during hi rpm excursions while auto-crossing. Go figure.
    Save the wave

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