[3] When the Yamaha XS 650 was launched in October 1969 it had one of the most advanced reciprocating engines in its class of large parallel twin motorcycles.

In 1969 only the Laverda 750S, and the Honda CB350, also launched that year, matched the XS 650's modernity of unit construction and SOHC valve operation.

1979 was the last year of the so-called "Standards" (an unofficial term commonly used to differentiate it from the "Special," which has pullback bars, a teardrop tank, and other differences in appearance).

There was a Special II (Two) model designation in 1978 (unknown) in 1979 (XS-SF-II), 1980 (XS-G), 1981 (XS-H) which were models with fewer chrome parts decals instead of plated plastic on the side covers as well as drum rear brake and wire wheels versus disc rear brake or cast wheels.

Upon removal of the compression release mechanism in 1974 due to updated starter, the square cover at the left exhaust valve was continued.

Brake modification notes: Handling differences on swapping rear wheels:- Prior to the XS-B model, the bike had a reputation for speed wobbles but adjustments to the 1975 setup overcame these.

A single overhead camshaft (SOHC) operates the XS 650's valves, whereas almost all contemporaries in its class have pushrod valvegear [citation needed].

The initial intake valve timing was changed in 1973 (EPA mandated) from a longer duration to a milder duration, this and other changes stifled the later models somewhat, the compression ratio also became lower over the years to match the milder cam timing, the flywheel is lighter than British contemporaries, this has the effect that the engine tends to pick up revs more rapidly when the throttle is opened quickly.

Pistons were lightened 20 percent along with connecting rods to reduce the reciprocating mass inside the engine.

Horizontal split of the crankcases offers the advantages of oil tightness through the elimination of vertical joints and one-step access to both the lower end and the gearbox.

The main bearings, transmission main shaft, clutch bushing, shifter fork guide bar, and rocker arms are lubricated by pressurized oil, whereas the rest of the engine is lubricated by “oil splash, this includes the big ends .”[6][7] Pre-1980 models use the twin 38 mm (1.5 in) constant velocity Mikuni carburetors that can be tuned by moving the needle clip position, or by replacing jets.

The motorcycle's average fuel economy was 51.4 miles per US gallon (4.58 L/100 km; 61.7 mpg‑imp)[9] Media related to Yamaha XS 650 at Wikimedia Commons