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Thread: coil issue/Kz900

  1. #1
    Vulture of The Western World Eric's Avatar
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    coil issue/Kz900

    I took my '76 Kz900 out today (first time since last year) and noticed it's not running quite right. After doing some checks, I discovered that one plug is not firing (or rather, being fired). The bike's running on three cylinders!

    Now, if you know these bikes, you know that the engine uses a pair of coil packs to run the four cylinder engine; each coil has two plug wires coming out of it and each of these wires fires two of the engine's four cylinders. The second coil takes care of the remaining two cylinders. I would have expected a failed coil to mean two dead cylinders - but here's where it gets funky: The suspect coil is firing one cylinder. As mentioned above, it has two plug wire leads coming out, each going to an individual cylinder. One is firing fer sure. (I popped a spare plug into the boot and observed the spark crossing the gap - and could also feel the pulse through my gloves!) But the other spark plug wire's not getting any juice; no spark at the plug.

    I'm assuming the coil's bad but thought I'd post here to get some feedback in case I am missing something....

  2. #2
    Administrator Ken's Avatar
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    Quote Originally Posted by Eric View Post
    I took my '76 Kz900 out today (first time since last year) and noticed it's not running quite right. After doing some checks, I discovered that one plug is not firing (or rather, being fired). The bike's running on three cylinders!

    Now, if you know these bikes, you know that the engine uses a pair of coil packs to run the four cylinder engine; each coil has two plug wires coming out of it and each of these wires fires two of the engine's four cylinders. The second coil takes care of the remaining two cylinders. I would have expected a failed coil to mean two dead cylinders - but here's where it gets funky: The suspect coil is firing one cylinder. As mentioned above, it has two plug wire leads coming out, each going to an individual cylinder. One is firing fer sure. (I popped a spare plug into the boot and observed the spark crossing the gap - and could also feel the pulse through my gloves!) But the other spark plug wire's not getting any juice; no spark at the plug.

    I'm assuming the coil's bad but thought I'd post here to get some feedback in case I am missing something....
    Eric, disconnect the spark plugs and the feed wires to the suspect coil.

    As one plug is firing I anticipate that one of the plug leads is open circuit. Measure the resistance between the two spark plug leads, I would expect something in the order of 20 to 25K Ohms. If the reading is infinity disconnect the spark plug leads from the coil and check across the coil output terminals, I would expect something in the order of 10 to 15K Ohms. If there is no continuity across the plug leads disconnect one lead and check from the 'bare' terminal to the plug cap. If there is continuity, probably in the order of 15K ohms or so then disconnect that lead and replace the other lead and check again. One of these checks should show open circuit and identify the faulty component. If all checks show OK then the fault, assuming the coil/lead assy isn't shorting to earth, has to be the plug.

    Ken.
    Die dulci fruimini!
    Ken.
    Wolds Bikers, Lincolnshire, England.

  3. #3
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    Quote Originally Posted by Eric View Post
    I took my '76 Kz900 out today (first time since last year) and noticed it's not running quite right. After doing some checks, I discovered that one plug is not firing (or rather, being fired). The bike's running on three cylinders!

    Now, if you know these bikes, you know that the engine uses a pair of coil packs to run the four cylinder engine; each coil has two plug wires coming out of it and each of these wires fires two of the engine's four cylinders. The second coil takes care of the remaining two cylinders. I would have expected a failed coil to mean two dead cylinders - but here's where it gets funky: The suspect coil is firing one cylinder. As mentioned above, it has two plug wire leads coming out, each going to an individual cylinder. One is firing fer sure. (I popped a spare plug into the boot and observed the spark crossing the gap - and could also feel the pulse through my gloves!) But the other spark plug wire's not getting any juice; no spark at the plug.

    I'm assuming the coil's bad but thought I'd post here to get some feedback in case I am missing something....



    I see four scenarios (most probable):
    • The plug is bad - grounded / fouled / cracked - pull it and see
    • There is a broken / bad connection to the plug
    • There is a bad connection / signal on the primary side
    • The coil pack is bad - I can't believe it, since it fires one cylinder
    Is it difficult to swap coil packs? If the problem follows the coil pack - then it must be the coil pack.

    If the problem doesn't follow the coil pack, then the signal to fire (on the primary side) isn't getting through.

    If you have a timing light (the clamp on inductive ones are especially nice) it's easy to tell if the plug is firing.

    Remember: If you can't fix it with a hammer - then it's an electrical problem!

  4. #4
    Senior Member DonTom's Avatar
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    Quote Originally Posted by Eric View Post
    I took my '76 Kz900 out today (first time since last year) and noticed it's not running quite right. After doing some checks, I discovered that one plug is not firing (or rather, being fired). The bike's running on three cylinders!

    My BMW has two spark plugs in series (two plugs in each cylinder). One fires as the other side has a "waste" spark.

    I too have had your problem. It was a bad spark plug. Plugs can get internal shorts putting all the spark voltage on the opposite plug. I would start by replacing ALL the plugs and then checking.

    It can be other things too, but I would bet on the plug. Replace them all when one goes bad. At least that's what I do.

    But you can move the plugs around to prove it's the plug before replacing any. See if you can move the problem to another cylinder.

    -Don-

  5. #5
    Vulture of The Western World Eric's Avatar
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    Quote Originally Posted by Ken View Post
    Eric, disconnect the spark plugs and the feed wires to the suspect coil.

    As one plug is firing I anticipate that one of the plug leads is open circuit. Measure the resistance between the two spark plug leads, I would expect something in the order of 20 to 25K Ohms. If the reading is infinity disconnect the spark plug leads from the coil and check across the coil output terminals, I would expect something in the order of 10 to 15K Ohms. If there is no continuity across the plug leads disconnect one lead and check from the 'bare' terminal to the plug cap. If there is continuity, probably in the order of 15K ohms or so then disconnect that lead and replace the other lead and check again. One of these checks should show open circuit and identify the faulty component. If all checks show OK then the fault, assuming the coil/lead assy isn't shorting to earth, has to be the plug.

    Ken.
    Thanks Ken,

    I will try to mess around with it some more today!

  6. #6
    Vulture of The Western World Eric's Avatar
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    Quote Originally Posted by DonTom View Post

    My BMW has two spark plugs in series (two plugs in each cylinder). One fires as the other side has a "waste" spark.

    I too have had your problem. It was a bad spark plug. Plugs can get internal shorts putting all the spark voltage on the opposite plug. I would start by replacing ALL the plugs and then checking.

    It can be other things too, but I would bet on the plug. Replace them all when one goes bad. At least that's what I do.

    But you can move the plugs around to prove it's the plug before replacing any. See if you can move the problem to another cylinder.

    -Don-

    Thanks Don - I will try that and see..

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