Suzuki GS series
From 1976 to 1987, Suzuki produced its GS line of motorcycles. These motorcycles had air-cooled DOHC engines with 2 valves per cylinder (as opposed to the GSX series which had four valves per cylinder), which spanned a wide range of cylinder capacities. It is considered to be a Universal Japanese Motorcycle.
When introduced in 1976, Suzuki offered the GS750, GS550 and GS400 models. Earlier Suzuki motorcycles were two-stroke.
As the flagship of the Suzuki line, the GS series was succeeded by Suzuki's GSX and then GSX-R motorcycles, which had oil (GSX) or liquid cooled (GSX-R) engines. However, the GS series is still produced as relatively affordable bikes, possibly used by learners in some jurisdictions, such as the GS500E (naked) and GS500F (fairing).
Links:
Classic GS resources
GS info & forum