Motorcycle Spark Plug Wires and Spark Plug Caps

Discussion in 'The Garage' started by ben2go, Nov 13, 2012.

  1. ben2go

    ben2go Moto Flunky

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    Jun 23, 2011
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    4,020
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    The victim Suzuki GS500E

    I need to replace the plug wires and spark plug caps.I know I need 7mm wires, but what resistance should the spark plug caps be?The bike uses a standard CDI ignition.Also,where do you purchase your plug wire and caps?
    #1
  2. bomber60015

    bomber60015 Anatomically Correct

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    you don't need 7mm . . . . . any diamter will work (it's mostly insulation) . . . . ..

    Caps, resstance . . . . .doesn't make a great deal differnece either.

    If you are wanting to make up your own set, grab the old set, and find the local hotrod shop . . . .. they'll have all sorts of high-zoot wire and caps . . . . .

    and not so high zoot, which will likely work just as well for your application.
    #2
  3. MrBob

    MrBob Certified Geezer

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    If you're using resistors in the wires make certain you're not also using resistor plugs.
    #3
  4. CycleDoc59

    CycleDoc59 Wrench Rider

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    Resistance can be in plug wires (carbon thread core), plug caps, and/or
    the spark plugs themselves.

    The primary purpose is reduction of stray electromagnetic interference;
    mostly with radio-type communications... But, those stray currents can
    cause backlash problems for on-board systems; CDI units, computers and
    the like.

    With the simple Kettering system (battery/points/coil) no resistance is needed
    except for radio interference. Not so with modern no points, high output systems.

    Also, resistance in plug wires depends on length and resistance per foot; change length,
    change wire type, change resistance.

    So, changing the original amount of ignition system resistance can cause
    running problems.
    #4
  5. ben2go

    ben2go Moto Flunky

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    This is exactly what I need to know.I just don't don't know what wires and caps to use.I know that my plugs are NGK DPR8EA-9 which I believe to be non resistor type plugs.The problem I am having is an occasional back fire on the left bank and an occasional dead left cylinder.Coils check out,CDI has been swapped to a known good unit,and crank triggers have been swapped out for known good units.Oh yeah,plugs have been swapped several times for new ones.Battery and R/R has tested good.I did the relay mod to power the coils directly from the battery,and ran new grounds directly from each coil mounting post to the battery negative terminal.All connections are clean and secure.The only thing I haven't changed is the coil wires and plug caps.7mm is the largest wire that will fit down inside my coils on the screw posts.
    #5
  6. CycleDoc59

    CycleDoc59 Wrench Rider

    Joined:
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    Oddometer:
    984
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    East Virginia
    With a couple of bikes that were ignition problem children, I've connected a timing light.
    Then taped the trigger down, and taped the light to the gas tank so I could see it flash
    while riding the bike.... Of course you have to stop and change leads from one plug wire
    to another. That pins down the grumpy cylinder. I also use a home-made spark tester
    almost daily, which adjusts so I can see how strong (long) and regular each spark is.
    Resistance-type plug wires can break internally if they are ancient and/or twisted all around.
    Plug wire size matters little unless the coil output is super high, as used in some
    very high performance hot rods/race bikes...

    If you already know which cylinder misbehaves, you may be able to swap plug leads between
    cylinders, or even coils/etc. If the problem moves along with the relocation..........

    There are other causes of backfires. Too lean, which can be a due to vacuum leaks, partially
    plugged jet or injector, valve out of adjustment or slightly burned, change of exhaust system and/or
    intake systems, leaky head gasket. Vacuum leaks are common between carburetors and heads.
    One recent Yamaha here popped and banged while running and when the air box was pulled back,
    the carb fell off. The rubber manifolds had perished. Easy to check for vac leaks with a propane
    torch (hey, not lit!)

    Point is, since swapping parts is pricey, you really need to pin down what's what before maybe
    buying more stuff you don't need...
    #6
  7. bomber60015

    bomber60015 Anatomically Correct

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    ^ This . . . . . . it may not be the ignition leads at all . . . . . .
    #7
  8. ben2go

    ben2go Moto Flunky

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    Oddometer:
    4,020
    Location:
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    I had done all the maintenance on my bike and about 300 to 500 miles later,I get this.So I tear the bike down and check everything.Everything checks out.So I strip down another bike I'm working on and use all the ignition parts on mine except the coils which have mysteriously went missing.I figure $10 to $15 for new plug wires and ends on a bike built in '89 isn't to much to spend.Napa has bulk 7mm plug wire for 69c a foot.I need two.They can also get plug caps in 1k,5k,and 12k resistance.
    #8