Honda Shadow VT500C spark unit compatibility??

Discussion in 'Motorbike Technical Discussion' started by mike, Mar 11, 2005.

  1. mike

    mike Guest

    Found a spark unit/igniter from a 1985 VT500C Shadow.
    Seller sez it fits '83-'86.
    I'd want to put it on my '83.
    But it's a single unit. My '83 has two seprate units.
    And the plugs have swapped sex.
    And only 4 used pins vs 5 on the 83.
    Looks like the trigger coil is grounded in the box on the '83
    and in the engine on the '85.
    Obviously, they're not plug-compatible.
    I'm thinking maybe I can rewire the plugs and make this work?
    Ideas? The correct unit is getting hard to find.
    Thanks, mike
    mike, Mar 11, 2005
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  2. mike

    mike Guest

    I've alredy stripped too much stuff off the parts bike. It's never
    gonna run again, so can't check the trigger waveform on it.
    I'm actually already using the ignition coils from the '85 parts bike.
    I think that's correct. My problem is determining which wire is which
    when I swap 'em.
    Sounds like you might have some experience here. I'll elaborate on what
    I've tried.
    I have the two spark units on the bike plus one more bad one that I
    replaced about 10 years ago. The bad one runs poorly at low speed, but
    it does run.
    I've swapped in all combinations of the three and can't tell any
    differences between the two that are currently on the bike.
    In the garage, I've hooked up a digital storage scope to look at the
    coil primary voltage and the secondary current using a current
    transformer. All three spark units seem to be putting out virtually
    identical waveforms...but I'm only looking at amplitude. Next, I need
    to hook up the trigger coils and look at phase.

    I've also tried instrumenting a spark unit on the bench. Drove it with
    a pulse generator, resistive load cause I didn't want to mess with the
    40KV on my bench. I can't tell any difference between a good one and a
    bad one. Changed all the electrolytic caps inside because the ESR
    mesured higher than I thought it should be, but that didn't help.
    Would be interesting to have a schematic for a spark unit. Wonder
    what's in that potted metal frame.

    My actual symptom is that the bike acts like it's running out of gas at
    about 45 mph after it warms up. Runs ok at 30 or 60 mph.
    I've even duct taped an oscilloscope to the tank and driven down the
    freeway in the dark looking at waveforms. I can't tell ANY difference
    in the spark waveforms between front and rear cylinder when it's acting up.

    "Sounds like a fuel problem," you say. Well, that's what I said
    too...but I've already changed everything in the fuel system up to and
    including the carburators and the gas tank. Changed the plugs and the
    ignition coils and wires and adjusted the valves and cleaned the air filter.
    I have an '85 Shadow parts bike.

    The only other item of concern is that the fron cylinder compression is
    only 60 or so. I've convinced myself that this couldn't be causing the
    intermittent system in only one small speed range.

    I've been messing with this on and off for a couple of years. The one
    consistent thing is that every time I mess with it, it seems to work ok
    for a day or so before returning to it's old habits. I've thought it
    fixed several times. I have no more hair to pull out. The spark unit
    thing is grabbing at the last straw I can think of, short of demonic


    Yep, this is the thing to try. My manual only goes up thru 1983, so I
    don't have the pinout for the '85 spark unit. Guess I'll have to open
    it up and reverse-engineer the wiring.

    Thanks, mike
    mike, Mar 11, 2005
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