Kasawaki KZ500/550 and the Kawasaki GPz550

Kawasaki Z500/Z550

This from Wikipedia for now. I'm still researching!

The Kawasaki Z500/Z550 series began with the 1979 Z500, a scaled-down version of the Kawasaki Z1R. It used a double-cradle steel (Norton featherbed-style) frame with a transverse-mounted air-cooled 4-cylinder DOHC (Double Over Head Cam) engine--a classic universal Japanese motorcycle.

KZ500,

This was the first of the line, with a 500cc DOHC engine, later bored out to 553cc's and also available as the 398cc Z400J (identical to the Z550). Early Z550 models (1980-1981) had a double disk brake in the front (The US model KZ550 had a single disk brake) and a drum brake in the rear, with a conventional swingarm using twin shock absorbers.

Later models (1982-1983) had improved brakes (twin disks in the front, with a single disk in the rear). The original Z500 (1979-1980) differed from the early Z400/Z550 models in having twin front and single rear disks.

Introduced in 1981, this was a slightly more aggressive version, with improved brakes, larger valves, more aggressive tuning, and a bikini fairing. The first model, the Gpz550D had slide carburettors, later models had CV carbs.

A monoshock rear swingarm was introduced in 1982 on the model Gpz550H.[1] The model was later substantially revised to the Gpz550A1 which had updated bodywork and revised valve timing increasing power to 68bhp.

The GPz 550 Forums are where you will find good advice from enthusiasts who know this bike inside-out.