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  1. #1
    CB750 New Member Dmatthews's Avatar
    Join Date
    Oct 2020
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    Oakmont
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    1

    Power to coil but not spark plugs

    Just bought a 1979 CB 750 F. New battery turns engine over. I tested power to the coils, each coil has power. I have no spark at any of the four plugs. I have looked at the wiring diagram, but not sure what would cause zero spark. Thanks to anyone for some help. I have over 20 early Honda scramblerís, which are much easier to diagnose.
    Thanks, Don

  2. #2
    CB750 Member fg1972's Avatar
    Join Date
    May 2019
    Location
    Australia
    Posts
    25
    Hi there,
    I assume you get +12v on the black wires when the ignition is on? If so, that's a start but to get "power to the coil" the other wires (blue and yellow) supply a ground pulse signal via the spark units which are triggered by the pickup mechanism each time the crank spins (similar to points). I'd check with a test light or multimeter that the pickups are working. Also check that primary and secondary resistance of the coils, I think the manual has the expected values but the main thing is that the resistances are not open circuit or shorted to ground.

  3. #3
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,233
    New (to owner) bike, may not even have the spark units..........sold to somebody else. They bolt down over the top of battery.

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