Very odd ignition problem and spark plug wire resistance values....

Discussion in 'Troubleshooting & Diagnosis' started by 70SSRS350, Jul 8, 2021.

  1. 70SSRS350

    70SSRS350 Veteran Member

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    My '70 with 350sbc / th350 is having very odd ignition issues, sometime the engine starts perfect and run well, sometime it starts very bad and die when I put in gear (any gear).
    When it starts bad, it sounds very odd, as it should have a completely wrong advance...

    I adjusted ignition advance: reduced, increased, with full vacuum, ported vacuum and no vacuum... but the result is still the same, sometime it works fine, sometime it works bad.

    I checked distributor (no play), coil, plugs and everything is fine.
    Then I checked spark plug wires.... I checked them for last because they looks perfect from outside, they are old but with just few thousand miles on them. I installed headers but also insulation socks on wires boots, so no burnt spot, no break or crack, wires are soft.

    I checked them with an electric tester/multimeter and....
    - ALL wires have no electrical continuity with my tester (?)
    - Most wires have about 1M ohm (1,000,000 ohm) resistance value (!)
    - One wire resistance is so high that it is not measurable by my tester (!)....

    Ok, wires have to be replaced, I think, but how can they work until now?
    From these values, it seems impossibile to me that these wires can take current to the plugs!
    From what I know, wire resistance should be under 10,000 ohm to be "acceptable" and these are 1 thousand worse!....
     
  2. tom3

    tom3 Veteran Member

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    Have to remember those wires are not wire. String with some mineral impregnated to the fibers. Draw moisture, vibrations, heat, age do a job on that stuff. And dealing with 25k (and up) volts and low current, lots of stuff going on there. Maybe a damp morning and it won't run for crap?
     
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  3. G72Zed

    G72Zed Veteran Member

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    I look at wires like tires, especially cars that do not get drivin much, the wires, "time out" before they wear out, even though they "look" new, like old tires with lots of tread, useless and can no longer do their job effectively.
     
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  4. xten

    xten Veteran Member

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    I've watched Kid test wires on his 8 sec Camaro. There are 3 different colors of plug wires on his car. lol
     
  5. COPO

    COPO Veteran Member Lifetime Gold Member

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    These are the best wires I have bought since 1973. 40 ohms/ft and that is an accurate number. Made in USA.
    E72D9BB3-3015-4230-BA0D-8EC94E39084D.jpeg E4FEBB7E-0EB2-4D3A-BE27-50DC4C6B0161.jpeg
     
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  6. fbody_mike

    fbody_mike Veteran Member

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    Your multimeter will not jump small breaks in the conductive core like a 30 to 40 KV pulse will. I'd bet the carbon core of your sparkplug wires are broken in many places. That high resistance will give your spark more inviting places to go instead of the intended sparkplug in a combustion chamber and depending on the environmental conditions would do so more often.
     
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  7. biker

    biker Veteran Member

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    Start it up at night and spray a mist of water over the wires. You may be able to see the spark arcing across breaks in the wires to the next most inviting ground.
     
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  8. cadillac_al

    cadillac_al Veteran Member

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    no mention of points maintenance, condenser could be going bad.
     
  9. Vettenick

    Vettenick Vettenick

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    Actual cylinder firing voltage is 8 to 12 thousand volts. Maximum resistance should not be higher than 12,000 ohms per foot. I think most good wires will check out 4000 to 8000 ohms per foot. If what your are measuring is correct, then your wires need to be replaced, But bad wires are usually consistently bad. One other mode of failure is wire leakage where they will arc to an external ground. In this case you will usually hear an intermittent snapping noise every time the wire leaks. It's easier to see in the dark.
    You did not mention whether it happens hot or cold. Have you checked the carb choke and choke pull off for proper operation?
     
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  10. Lowend

    Lowend Administrator. .a car, a man, a maraca. Staff Member Lifetime Gold Member

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    The wires are DONE
    Sparkplug wires are measured in ohms/foot.
    If you see a value over ~400ohm/foot the wire is bad. Point of reference, new MSD 8.5mm wires are 50 ohms/foot
    Until those are right, there's really no point in trying to diag anything else. FYI - I dislike the plug-boot covers. All products like that still require an air gap between your boot and the header tube (inverse square rule). As soon as things touch, the heat conducts and you are cooking stuff. Most of the time the boot covers use up what little space you have and end up touching.
    When you redo the wires, use these boots instead and leave the sleeves off
    https://www.holley.com/products/ignition/spark_plug_wires/spark_plug_wire_accessories/parts/8852

    The next step is to put a timing light on the car, see where the initial spark advance is (should be 4-8 deg BTDC in most cases)
     
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