TOPIC:

While running, the RPM needle started to bounce, and then it dropped to 0. Volt check works ok. If I press the button needle goes up.
Everything else works fine. All the lights and indicators are functional.
So, there must be a problem with the pulse lead. Where does this wire start?

Here is a link to the real Kawasaki service manual for your bike (in case you don't already have it.)  Inside are wiring diagrams and a tacho troubleshooting chart to guide you.

www.kzrider.com/modules/ServiceManuals/Kz400500550_79to83.pdf

Nessism post=846030
Here is a link to the real Kawasaki service manual for your bike (in case you don't already have it.)  Inside are wiring diagrams and a tacho troubleshooting chart to guide you.

www.kzrider.com/modules/ServiceManuals/Kz400500550_79to83.pdf

Thank you Nessism, very, very much. 
That was exactly what i needed. I had another one of wery poor quality and questionable content.

I've fixed my tacho.

Now, there is just one thing to do and the bike will be mint.

Oil warning light. I gues that's electrical problem too. I'll check the leads and if that doesn't help I'll try to clean a sender/switch.

Anyway, thank you!