YAMAHA SX750 3CYL 1978

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

 

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
butterflies.

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.

 

thanks guys.your great to help me. ill post what the problem is when i
find it, lucas

http://www.minibite.com/america/malone.htm

 

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
engine.

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
afterward.

-gc