Manufacturer yamaha XS650

The XS650 was a standard motorcycle made by Yamaha from 1968 to 1985. The XS650 began with the 1955 Hosk SOHC 500 twin. After about 10 years of producing 500 twin, Hosk engineers designed a 650cc twin. Later the Hosk was acquired by Showa Corporation, and in 1960 Yamaha had bought Showa with Hosks early design of 650cc twin.

Manufacturer Yamaha Motor Company
Also called XS-1, XS-2
Parent company Yamaha Corporation
Production 1968–1985
Class Standard
Engine 654cc, 4-stroke, parallel twin, air-cooled, SOHC, 2 valves per cylinder
Power 53 bhp @ 7000 rpm
Ignition type

Contact point-pre 1980
CDI-post 1980
Transmission 5-Speed sequential
Frame type tubular steel
Weight 450 lb (dry)
Fuel capacity 3 U.S. Gal


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 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. Due to the fact that the flywheel is lighter, the engine tends to pick up revs rapidly when the throttle is quickly open.

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 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 pistons.

Post-1979 models use smaller 34mm 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 34mm 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.[clarification needed] That is true for the later model, '80-on, XS[850] triples, Yamaha's three cylinder 750/850 model.