new to me 1976 KZ 900, wires go where?

Ok,  getting somewhere.
You'll need 3 hands on this part.
key on ignition on
The black/yellow wire still needs to be connected to the negative battery post.

The small black wire at the relay.. Find it's connector and disconnect it. The section that goes to the relay attach a length of wire to it that can go to battery positive. At this wire joint put some tape so it doesn't touch ground accidentally.
You will just touch this wire to battery postive.
no need to do the above. sorry i couldn't edit fast enough on my phone.

Meter
Black probe on battery negative.
Red probe on the large relay bolt that goes to starter. Not the large bolt that goes to battery.

You will note the voltage without touching the small black wire to battery positive.

Then touch the wire to battery positive and note the voltage.
note voltage
press start button and note voltage


Move the red probe to the unconnected black/yellow that does not go to the relay.
What is the voltage at this connector.

ok @F64
results below....
Meter DC V
Black probe on battery negative.
Red probe on the large relay bolt that goes to starter. Not the large bolt that goes to battery. 20.0v

You will note the voltage without touching the small black wire to battery positive. OK, not touching the wire the reading is 20.0v, when touching, hear a click and voltage 11.8v
Then touch the wire to battery positive and note the voltage.(info above)

The key and ignition must be on for this next test.
Move the red probe to the unconnected black/yellow that does not go to the relay.
What is the voltage at this connector. - 12.07v

Ok,
The relay works.
Your starter should be able to turn now but we won't mess with that till we get your main ground wire for the bike fixed.

That unconnected black yellow wire that you just measured 12v at.
You have to trace that wire.
Someone connected your main ground wire to 12v positive.
It may be in the headlight bucket where they put a black/yellow connector into the wrong connector.
Look in the headlight bucket first.
You will be looking for all of the black /yellow wires in there.


I would have the battery in a place where you can disconnect the negative terminal fast.
This bike is wired badly. If you start to see or smell burning, you want to disconnect the negative battery cable quickly.
 

Ok, @F64 thank you for the help.  let me know when to put those wires back together.
I am attaching photos below of the headlight bucket, it's a bit of spaghetti.
It looks like the main blk/yel wire coming into the bucket from the main loom goes to the headlight wiring and a turn signal ground.


 

Check to make sure this yellow black wire connector near the battery is connected to the negative battery terminal.

Schematic courtesy of Mikaw

Hi @Mikaw,
The only wire going to the negative side of the battery is the large black battery cable that is bolted to the side of the engine case. I don't see any other wires near by that would appear should be connected to the battery.

There needs to be a black/yellow wire connected to the negative battery cable.
It may be loose under a side cover.

You could as a temporary fix connect the small black/yellow wire connector at the relay back together but leave the wire connected to the negative terminal attached.
 

@Mikaw, @F64, attaching a photo
I didn't see this since I wasn't looking but the negative battery cable at the engine block does have a blk/yellow wire.(Maybe my Neg battery cable is flip/flopped) Appears to travel up the frame to the main loom.


 

@Mikaw.
The schematic you provide is very blurry. I am trying to read the colored wires and where they go. Is there a clearer version online I may be able to access?

Yeah, good old schematics.
Not sure about the colored version.
I got this schematic from the first page of your post. Mikaw posted 2 schematics.

Is that black and yellow wire attached to the engine block?

Hi @F64,
It appears it was flip flopped and yes it is attached to the engine block, I can flip it around to look correct. From the engine block The blk/yellow wire goes up the frame with brake light wires to another connector into the wire loom.

Put a meter on the wire at the taillight connector.
Key on / ignition on DCV

Red probe at the black/yellow taillight connector.
Black probe on the engine case.
It should be 0.0v



The ground at the engine case should be fine.
But may be best to flip the battery cable as your engine case may not be grounded.