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HEI and exhaust backfiring

Discussion in 'Early CJ5 and CJ6 Tech' started by garage gnome, Apr 5, 2011.

  1. Apr 5, 2011
    garage gnome

    garage gnome ECJ5 welder

    Western MA
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    Jun 19, 2009
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    3,092
    Ever since I put the HEI in my :v6:, it keeps backfiring through the exhaust when I'm going down a hill and I am using the compression to slow the jeep. I'm either thinking that my spark plug gap is too small (45 thousands) or I need to adjust my timing (at 5° before TDC). The carb seems to be adjusted fine. Ideas, opinions, and experiences?
     
    garage gnome, Apr 5, 2011
    #1
  2. Apr 5, 2011
    bkd

    bkd Moderator Supreme Staff Member 2022 Sponsor

    K-Town Tenn.
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    Apr 29, 2007
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    thinking I ran my plug gap at 55 with hei.......worth a try
    Jim
     
    bkd, Apr 5, 2011
    #2
  3. Apr 5, 2011
    garage gnome

    garage gnome ECJ5 welder

    Western MA
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    I'll try re-gaping them.

    I also would like to add that the plugs, wires and air filter are all new to eliminate weak plugs and a plugged up air filter making it run rich.
     
    garage gnome, Apr 5, 2011
    #3
  4. Apr 5, 2011
    Warloch

    Warloch Did you say Flattie??? Staff Member

    Falcon, CO
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    I went to the E3 plugs (no gap there) and am running 12 at my altitude. The plugs are expensive, but have seen nice things (power and fuel econ) in everything I have put them in.
     
    Warloch, Apr 5, 2011
    #4
  5. Apr 6, 2011
    w3srl

    w3srl All-around swell dude Staff Member

    Port Orange, FL
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    Nov 6, 2002
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    4,275
    You might want to try advancing the timing just a tad, and look at the idle mixture as well. Late timing and excessively rich idle mixture will put unburned fuel into the exhaust where it can light off as a backfire.

    Another thing to look at is exhaust leaks in the header-to-engine gaskets and also at the collectors. A leak here can be minor enough to not cause issues under acceleration, but when decelerating a leak will tend to suck enough oxygen in to cause a detonation in the exhaust. Sounds odd, but it's worth looking at it.
     
    w3srl, Apr 6, 2011
    #5
  6. Apr 6, 2011
    garage gnome

    garage gnome ECJ5 welder

    Western MA
    Joined:
    Jun 19, 2009
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    I re-gapped the plugs to 55 thousands and that took away about 1/2 of it. Then I brought the timing to 7° before TDC, where I used to run it at. That took away a little bit more. I still have a little left. When I pulled the plugs they were a dark dirty tan color. Not quite the perfect golden tan color they used to be. This means I'm running a little too rich, right?

    I don't see any evidence of an exhaust leak. Someone used red RTV between the headers and the exhaust gaskets.
     
    garage gnome, Apr 6, 2011
    #6
  7. Apr 6, 2011
    Warloch

    Warloch Did you say Flattie??? Staff Member

    Falcon, CO
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    Touch too rich yet, yup.
     
    Warloch, Apr 6, 2011
    #7
  8. Apr 27, 2011
    garage gnome

    garage gnome ECJ5 welder

    Western MA
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    I figured out this problem. I was getting POed at this thing one day, and for the heck of it, I took the vac advance off and plugged it up. No more backfiring! :) So, why would the vac advance cause it to backfire? Advanced the timing too much? I guess the mechanical advance is equivalent to the mech. + vac advances on my old distributer.
     
    garage gnome, Apr 27, 2011
    #8
  9. Apr 27, 2011
    Warloch

    Warloch Did you say Flattie??? Staff Member

    Falcon, CO
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    That's what I would suspect. Alot of them have the 231 setups and they can have more of a curve than is needed on the 225s.
     
    Warloch, Apr 27, 2011
    #9
  10. Apr 28, 2011
    noahlon

    noahlon Old Fart

    Pine Ridge, AR
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    Dec 22, 2010
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    Installed a HEI on my 225 last week. Bosh plugs at 45, timing at 10. Eery thing works great.
     
    noahlon, Apr 28, 2011
    #10
  11. Apr 29, 2011
    Jim302

    Jim302 Banned

    Morrisville PA
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    Jan 4, 2011
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    Did you try using ported vacume? That would be the equivalent of plugging it when off the throttle.

    I hate to sound redundant, but do you have the correct #1?
     
    Jim302, Apr 29, 2011
    #11
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