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Yamaha XS 650 (1968–1985) specifications

2014-04-04 Read: 1 092x

Engine

Like its 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.

The XS 650s valves are operated by a single overhead camshaft (SOHC) whereas almost all contemporaries in its class have pushrodvalvegear.

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 intake valve opens 47 degrees BTC, closes 67 degrees ATC, yielding intake duration of 294 degrees, exhaust duration on 281 degrees, and an overlap of 88 degrees. Because the flywheel is lighter than British contemporaries, 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, crank pins, 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.”

Carburetion

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. 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 34 mm (1.3 in) 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 34 mm (1.3 in) CV carb needles only have one fixed clip position.

Ignition

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 ignitionsystems.

Performance

Cycle World tested the XS650 in March 1979. It ran the standing-start quarter mile 13.86 seconds with a terminal speed of 96.05 mph (154.58 km/h). The motorcycle's average fuel economy was 51.4 miles per US gallon (4.58 L/100 km; 61.7 mpg-imp)

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Yamaha XS 650

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