Back in 1978 when Yamaha first created the SR500 it was intended to be a Japanese replacement for British thumpers like the Manx Norton – yet Yamaha also made it a bike for entry level riders by building the SR series to be easy to ride and easy to maintain. The Yamaha SR400 and SR500 really begin with the engine fitted to the XT500 enduro/adventure motorcycle made from 1975-1981. The XT500 was a bike built for lively performance and had rugged, simple construction. It was a bike that was successfully campaigned in the Paris–Abidjan-Nice and then the Paris–Dakar Rallies establishing itself as a credible adventure machine.
Yamaha took the engine used in the XT500 and created two versions of it, the longer stroke 499cc capacity version and the same engine with a slightly shorter stroke giving it a just under 400cc capacity and an interestingly different personality for the SR400. So it was pretty much the same engine used in the XT500 and the SR500 and the shorter stroke engine for the SR400 initially made for the Japanese Domestic Market. The SR400 was first shown in Japan in 1978 and was restricted to the Japanese market, its engine capacity having been reduced to comply with Japanese motorcycle licensing regulations.