Yamaha XS 650

The XS650 is a mid-size motorcycle made by Yamaha Motor Company. The standard model was introduced in 1968 and produced through 1979. The "Special" cruiser model was introduced in 1978 and produced through 1985. The XS650 began with the 1955 Hosk SOHC 500 twin. After about 10 years of producing 500 twin, Hosk engineers designed a 650 cc twin. Later the Hosk was acquired by Showa Corporation, and in 1960 Yamaha had bought Showa with Hosk's early design of 650 cc twin.^[1]

When the Yamaha XS 650 was launched in 1968 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 1968 are either unit construction with a vertically-split crankcase or pre-unit construction with separate engine and gearbox. The XS650s engine was used in AMA Professional Dirt Track Racing by national champion Kenny Roberts. In 1968 only the Laverda 750S, also launched that year, matched the XS 650's modernity of unit construction and SOHC valve operation.

Model history

The 1970 model was designated the XS-1.^[2] Subsequent models were XS-1B (1971), XS-2 (1972), then TX (1973), TX-A (1974), XS-B (1975), XS-C (1976), XS-D (1977), XS-E (1978), XS-F (1979). 1979 was the last year of the so-called "Standards" (owner's term meaning opposite to the 650 "Special," which had pullback bars, teardrop tanks, and other custom features). The Es and Fs also came in Special form: XS-SE (S for Special) and XS-SF. From then on they were Specials only to XS-SG, XS-SH, XS-SJ, XS-SK. There was a Special II (Two) model designation in 1979 (XS-SF-II), 1980 (XS-G), 1981 (XS-H) which were models with fewer chrome parts and drum rear brake (1979, 1980) or wire wheels (1981) versus disc rear brake or cast wheels.

The first two model years (XS-1 and XS-1B) were kick start only, with an electric starter added from the 1972 model year on. This had a compression release added to the front left exhaust tappet cover resulting in a square versus triangular cover found on the other exhaust and intake covers. Upon removal of the compression release mechanism in 1974 due to uprated starter, the square cover at the left exhaust valve was continued.

Brake modification notes:

• Drum fronts on early models cannot be changed.
• Pre '77 & post '77 fronts have different (offset) brake discs, the wheels swap but not the discs.
• Drum rear wheel into rear disc frame needs the rear drum frame swingarm too.
• Disk rear wheel into rear drum frame needs the rear disc frame swingarm and also needs brake master-cylinder lugs welded to the frame.

Handling differences on swapping rear wheels:-

• Standard rear tire is 110/90-18. Special rear tire is 130/90-16. Because the Standard tire is narrower it will tend to steer a bit more quickly. Because the Special tire is wider it will tend to be a bit steadier in a straight line. Overall gearing is not changed from the Standard, as the two tires are essentially the same outside diameter.

1976 models had the front brake caliper moved to the right fork leg, behind the fork as opposed to in front of the fork. This placed the brake caliper nearer the axle center-line, requiring slightly less effort to steer.

Mid-'77 the front forks had a major redesign, fork tube diameter increased from 34 to 35 mm (1.4 in) and internals were changed (although this also holds true for various years of the same tube size). The entire fork assembly (with triple tree) will swap either way but fork parts are not equivalent. Also the brake caliper changed from a 48 mm (1.9 in) dual piston cast iron design for the 34 mm (1.3 in) fork to a 40 mm (1.6 in) aluminum single piston floating caliper for the 35 mm (1.4 in) forks. The brake caliper mounting lugs on the fork sliders are of different spacing for the 34 mm (1.3 in) and 35 mm (1.4 in) forks so the calipers can't be swapped.

End of production

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 were left over due to overproduction and an economic recession and brand new 1982 and 1983 models could still be purchased in 1987 at some dealerships.

Motorcycle model ID table

^[3]

Design

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 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 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.”^[4][5]

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.

As previously noted, of '81-'83 models using; Hitachi carburetors with all jets pressed in place. That was not correct for the XS650s. That is true for the later model, '80-on, XS[850] triples, Yamaha's three cylinder 750/850 model.^[6]

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. And advance mechanism is used to retard the timing for easy starting and smooth idle. Post-1979 models use electronic ignition systems.

Performance

• Standing-start quarter= 13.86 sec at 96.05 mph (154.58 km/h)
• Motorcycle average gas millage=51.4 mpg^[7]

See also

• Yamaha XS750 Triple Special
• Kawasaki KZ650
• Honda CB650

References