What country are you in? I'm guessing you're somewhere in Europe.
A KZ750 would be a USA or North American model. A Z750 would be Europe or elsewhere.
The 82 Kawasaki 750 Twin service manual may cover your model,
and it may have a European wiring diagram as they are usually different from USA wiring diagrams.
In the USA there was an 82 KZ750-M1 CSR Twin (chain drive). later there were the 83-84 KZ750-K1/K2 Ltd Twins (belt drive).
Note: the KZ prefix was only used in the USA or North America. KZ has some sort of nazi connotation, so they didn't use that in Europe. The letter & number suffix is used to differentiate between models & years. A 750-M1 is a Twin. that was the only year for the model, but if there had been an 83 750-CSR, it would have been designated an M2. M1 is the 1st year of the model & M2 would have been the 2nd year if it had been made.
See model application chart on back cover. The KZ750-S1 & Y1 are listed. I don't know anything about those two.
Per KZr member, steell:
76-79 KZ750 were the "B" model, single disc front and rear with wire spoke wheels.
1980 KZ750G, pretty much the same as the "B", but with rear drum brake and mag wheels.
1981 model 750 twin doesn't exist.
1982 KZ750M1 CSR, same motor as the "B" and "G", with air injection into the exhaust, with wire wheels and rear drum brake.
1983 KZ750K1, same as M1 except for belt drive.
1984 KZ750K2, Canada only, same as K1.
Per KZr member,TexasKZ:
The K1 and K2 (LTD) had mag wheel
I was going to guess Sweeden as I think someone here on KZR mentioned that the S1 was sold there.
Try comparing your bike to the KZ750-M1 CSR. My guess is that it is almost the same as the S1.
The csr had wire spoked wheels. see photo on front cover of manual above.