Yamaha XS 650 мотоциклінің тарихы мен ерекшеліктері

Yamaha motorcycle
When the Yamaha XS 650 was launched in October 1969 it had one of the most advanced engines in its class of large parallel twin motorcycles. The engine and gearbox are unit construction with the crankcase split horizontally for ease of assembly whereas almost all contemporaries in its class in 1969 are either unit construction with a vertically split crankcase or pre unit construction with separate engine and gearbox. The XS650's engine was used in AMA Professional Dirt Track Racing by national champion Kenny Roberts. 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.

The XS 650 was produced until 1985. The last model year was 1983 in the United States, with Canada, Europe and other markets continuing into 1984 and 1985. However, many US models remained unsold for some years due to overproduction and an economic recession and brand new 1982 and 1983 models could still be purchased in 1987 at some dealerships.

Like the contemporaries in its class the XS 650 has a 360° crank angle. This provides an even firing interval between the two cylinders, but also generates some vibration caused by the two pistons rising and falling together. This vibration is particularly noticeable at idle. Because of a long acknowledged vibration issue, starting with the 1974 models, vibration damping modifications were made to the engine and handlebar mountings. The XS 650s valves are operated by a single overhead camshaft (SOHC) whereas almost all contemporaries in its class have pushrod valvegear
The 360 degrees crankshaft uses three roller bearings and a ball bearing. The camshaft uses four ball bearings, and rolling bearings are used throughout the rest of the engine. Connecting rods turn on needle bearings. Since the engine is SOHC, there are no pushrods to operate the valves. The camshaft gets its drive from a single row chain running from the center of the crankshaft. Chain tension is maintained by a spring loaded guide, which also takes up unnecessary slack. 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. During the later developments of the engine compression ratios were lowered, then raised. Pistons were lightened 20 percent along with connecting rods to reduce the reciprocating mass inside the engine. Aluminum pistons are slightly domed with valve pockets. Pistons have three rings installed, two compression and one oil control ring. 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. Oil pressure is provided by the trochoidal pump, driven by a steel spur gear off the crankshaft. 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 .”

Pre 1980 models use the twin constant velocity Mikuni carburetors that can be tuned by moving the needle clip position, or by replacing jets. In the carburetors the velocity of the fuel mixture through the venturi, regulated by the opening of the butterfly valves and engine speed, causes a pressure difference between the top and the bottom of the carburetor pistons. This pressure difference raises and lowers the carburetor slides, increasing or decreasing engine output accordingly. Post 1979 models use smaller Mikuni CV carbs with needles that seem to be listed in parts menus as being 'fixed' position,(in other words a needle that may only have one clip position). The pilot and main jets can be changed for different sizes. If the CV carb needles only have one fixed clip position.

The models up to 1979 use points ignition. Two sets of points are located on the upper left of the cylinder head. On the right side cylinder head, an advance mechanism is located. An advance mechanism is used to retard the timing for easy starting and smooth idle. Post 1979 models use electronic ignition systems.

Cycle World tested the XS650 in March 1979. It ran the standing start quarter mile 13.86 seconds with a terminal speed of The motorcycle's average fuel economy was