1978 Mercury 200 20 hp, electric start

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  • Buccaneer
    Buccaneer

    US Member - 1 Year
    Replies: 4065
    Topics: 980
    #216363

    The subject motor came with a recent boat purchase. I decided to check it out before I sell
    it, hopefully as a running motor. I was told it ran a couple of years ago, but in looking at the
    wiring, I have my doubts.
    What the cause of of all the insulation disintegrating on Mercury’s wiring in that era?
    I decided to try to patch up the wiring before I even try to crank it over to check for spark
    or compression.

    I found one wiring diagram that’s “close”, but it’s not showing the rectifier that mine has.
    Wouldn’t they all need one in that era to charge the battery?
    There’s two wires from the stator going to the top of the rectifier, then one going to
    the positive 12 v from the battery.

    I pulled the flywheel and don’t see any bad wiring right at the stator, but the rest of the internal
    wiring harness was a mess!

    Anyone have a digital repair manual for this model?
    The serial number is in the 49XXXXX range.
    I’m used to working on the old magneto units!
    Thanks.

    Merc-200-Wiring-diagram

    DSCN9465

    DSCN9469

    DSCN9466

    Prepare to be boarded!

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    billw

    US Member - 2 Years
    Replies: 1274
    Topics: 41
    #216380

    Yeah, the electric starts had those rectifiers. That’s the right one. On the insulation, it really is a shame how it goes bad, because originally, it seemed like really good-quality, flexible wire.

    Long live American manufacturing!

    Buccaneer
    Buccaneer

    US Member - 1 Year
    Replies: 4065
    Topics: 980
    #216509

    Update – I patched all the wiring up on the internal harness….. pretty much replaced
    all but the large battery leads. Hooked up the external harness today to a battery,
    and the starter works, the motor spins over, with 116 psi on both cylinders,
    and it has spark……. at least on my conventional test plugs. I did not try
    the surface gap plugs that came with it.
    That’s a good start, and a surprise after seeing all that wire crumbling apart.
    I took the motor off the stand and threw it on the back of the boat, being the
    motor is a long shaft and too long for the stand it was on.
    Sometime yet this week I hope to hook the muffs up to it and see if it will run,
    if’n the carb isn’t gunked up.
    I need to check the lower unit yet. Do the 1978 Mercury’s use conventional marine
    80 / 90 weight gear oil?

    If it runs, what would be an average asking price for this MN Mercury?
    Thanks.

    Prepare to be boarded!

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    dave-bernard

    US Member - 2 Years
    Replies: 1185
    Topics: 14
    #216526

    yes on the oil. don’t forget to replace the impeller.

    Buccaneer
    Buccaneer

    US Member - 1 Year
    Replies: 4065
    Topics: 980
    #216529

    Thanks Dave. Will see if it runs so I can sell it for more than parts, and if so,
    will put a new impeller in it so it’s “good to go” for the next owner.

    Prepare to be boarded!

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