new to me 1976 KZ 900, wires go where?

Red probe on the positive battery post.

Black probe on the black/yellow connector that you just connected. Leave the connector together. Just squeeze the probe under the clear boot.

What is your voltage?
 

Red probe on the positive battery post.

Black probe on the black/yellow connector that you just connected. Leave the connector together. Just squeeze the probe under the clear boot.

What is your voltage? 11.86, key on 11.64.

Do you hear a click  from the relay    when you press the start button?

Did you reconnect the small black wire connector from the relay as well?

Hi F64
YEAH!!!!!!  Per above I reconnected the small black wire!!  and holy smokes!!  The starter is working!! So what is up? Is my main battery negative cable bad?

Haha,
Nah.
The engine case wasn't grounded.

Your main ground wire... Black yellow wire was on the engine case. The engine case wasn't grounded. So when you added the ground wire to the case..the black /yellow wire was grounded again. Then a lot of stuff should work now.

WOW!!!! thanks for all the help from everyone. My next task, changing the oil and figuring out the wiring. plus the coils and many other things.

@F 64
ok, so what do I do with the wire I have grounding it? What is proper engine ground?

If the bike has been sitting I’d pull the plugs and put a little marvel mystery oil in and let that soak before you crank if over to much. Also I thought you were getting odd reading at the coil primary. That should be tracked through before start is attempted. What does the tank look like? Is there a fuel filter inline and is it clear. Lots of things to check before a start attempt


before start charge the battery fully
 

I'd put a 12 AWG wire going from battery negative to the engine case.

I have a 6awg on mine but I'm a bit crazy.
12awg should be fine.

Thanks @Mikaw, F64
The tank is in great shape, the petcock had been removed and all the gas drained.
The engine turns but as you said I haven't turned it over much. Yes I will be replacing the coils if needed. Plugs look new. Plug wires look shabby. Lots of things to do. Carbs need cleaned. You and F64 and others have helped me get it powered up and the starter working. It was a dead bike with a new battery when I first posted. So I need to get the wiring tidied up and then continue the restoration. Paint is faded but original ,which I can buff out. I have a new Kerker header for it. New shocks.  Everything else is in decent shape. Some rusty chrome here and there.
 

Keep at it.
You got past a huge part of it.
Just keep moving on it.

@F64.
So I should have a second ground to the engine case from the battery? What purpose is the large black negative cable to the engine case? What if I replaced it, it's a bit twisted up and seems like original, maybe its corroded?