YAMAHA SX750 3CYL 1978

Discussion in 'Motorbike Technical Discussion' started by ds549, Mar 23, 2007.

  1. ds549

    ds549 Guest

    i have the above bike and need to know if anyone can tell me how
    many amps the starter is supposed to draw. it is drawing 50 amps with
    new good battery. battery voltage stays above 11 volts when cranking,
    but my posative wire at the ignition ignightor is dropping to 4-5 volts
    when cranking, so ,no spark. i can put a charger on the battery and she
    will fire right up but with charger conected and cranking,my posative
    wire on the ignightor is about 8-0 volts. any help would be appreciated.
    i have measured voltages while cranking along the system to the starter
    and is 11 v when cranking. so,i need to find why ignightor voltage is so
    low..battery voltage is 14 volts when running.. lucas
    ds549, Mar 23, 2007
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  2. It's an XS750, not an SX750
    That's about right. 50 X 11 = 550 watts = 0.55 KW, which is about
    2/3rds of a horsepower. My Suzuki GT750 had a 0.5 KW starter.
    The starter circuit, from the battery through the solenoid, through
    the starter to ground and back to the battery is a whole different
    parallel circuit, but what happens with voltage on the starter circuit
    as the starter cranks the engine should not reduce the
    voltage at the ignitor box any more than it reduces
    battery voltage unless you have some strange electronic control device
    in the circuit.

    You might want to look at the electrical diagram
    at www.partsfish.com to see if that model had a
    headlight cutout relay that is supposed to turn the headlight off to
    reduce current draw while you're
    cranking the engine.

    Maybe your ignition switch is all corroded and burned up inside. I had
    that happen on two different vehicles. That would really drop the
    voltage to the ignitor box quite a bit.

    Also, look at all the wiring in the ignition circuit for blackened
    insulation or melted connectors. You might want to do an ohmmeter
    check to see if you have low resistance through the kill switch.

    Then there is your starting procedure. It's really not necessary to
    crank the engine with the starter for more than five seconds at a
    time. My GT750 2-stroke triple would start within about 2 turns of the
    crankshaft, your XS750 4-stroke might take 4 or 5 turns when the carbs
    are clean.

    You have the modern diaphragm carburetors with cold enrichment devices
    instead of chokes, and you're not supposed to twist the throttle when
    trying to start the engine.

    Full "choke" opens a little air valve in the side of each carburetor
    and gasoline is sucked right out of the float bowls and the gasoline
    and air bypass the throttle butterfly.

    Any attempt to "help" the engine start by twisting the throttle just
    defeats the cold enrichener by lowering the vacuum downstream of the

    If a motorcycle has been stored for a while, it actually helps to turn
    the master idle knob on the carburetors down so the throttle
    butterflies are closed all the way when you try to start the engine.

    When the engine fires, you can then hold the throttle slightly open
    until the engine warms up and you can readjust the master idle knob.
    Potage St. Germaine, Mar 23, 2007
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  3. ds549

    ds549 Guest

    ds549, Mar 23, 2007
  4. ds549

    Gene Cash Guest

    I had the problem with my GS1100GL, and it turned out there's one ground
    wire from the frame to a small rusty 8mm or 10mm bolt on the back of the

    The connection was horrible! Something on the order of many K-ohms when
    I started working on it. Charging and starting was much much better

    Gene Cash, Mar 23, 2007
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