Yamaha XS-1B 650

Breaking With Tradition To Extend An Older One, Yamaha Comes Up With A Well-Disguised Racing Engine In A Sporting Big Bore Roadster.

YAMAHA'S NEW BIG BORE will be likely to cause confusion to innocent bystanders. From a distance it resembles several other machines that follow the almost classic 650-cc or 750-cc vertical Twin pattern.

It may seem strange that Yamaha-breaking with tradition to build its first four-stroke machine-would follow a path already beaten. Starting from scratch they could have opted to build a three- or four-cylinder super bike. But the situation in both the Japanese and U.S. markets made the 650 Twin a wiser choice. For one thing, the big domestic market in Japan is becoming more affluent.  Like many Americans, they buy up the medium displacement machines in great droves, but hanker after that day when they can own a "real" (translate that as "big displacement") motorcycle.  The jump in cost and size from a 250 or 350, to it 650 Twin, is not quite so big as to a larger one.  Expanding markets in S.E. Asia are also getting ripe for a 650.  In the U.S., where the superbike battle is raging, the XS-650 slips neatly into that less costly "in-between” area.

It's not a bad move, as the vertical Twin has been a popularly accepted form of sporting transportation in the U.S. for about two decades.  As a design, it is pre-sold.  It is reasonably compact and light, and lends itself to sporting applications, both in everyday road use and on the race track.  

There is no question that Yamaha has achieved the classic Big Twin "feel."  Start the XS-650, sit on it and close your eyes and you could be sitting on any one of four British Twins. Seating position, handlebars, height, general balance and weight distribution, even the sound of the 360-degree alternate firing crankshaft arrangement, recall another country and a tradition other than Japanese.

But there are differences that are quite Japanese.  Flywheel effect is lighter, and the engine picks up revs quite rapidly when the throttle is blipped.  The five-speed gearbox shifts on the left.  The XS-650 is delivered in a tractable state of tune, and doesn't really need the five ratios.  But the gearbox, the single overhead camshaft, and the overall construction of the engine beg the race tuner's hand.  The machine is robust, laid out for rapid access to its internals, and ready to be stretched.

It may seem rather rude to call the engine layout of the XS-650 "conventional," for the machine is the most sophisticated 650-cc Twin commercially produced.  But conventional it is, in Japanese terms.  And this normalcy is actually a desirable attribute.  The practice is already proven.

For example, the crankcases split horizontally, offering the advantages of oil tightness through the elimination of vertical joints and one-step access to both the lower end and the gearbox.  The 650's four-main-bearing crankshaft, made up of four separate flywheels, recalls the practice established by Honda in its smaller Twins. Couching that great rotating mass, in so many main bearings virtually eliminates that old parallel Twin bugaboo-crankshaft flexure at high rpm. This may seem unnecessary in a machine with a 7000-rpm power peak, but it should be evident that the machine can be turned much tighter with safety.  This leaves much room for annual development of production models, as well as any optional power kitting. And it should make Yamaha's active racing department mighty happy.

Alternate 360-degree firing order is used, giving even firing impulses, and that familiar husky sound that has been the trademark of the big Twins for years. A splined hub, connecting the two sets of flywheels, incorporates a sprocket to drive the overhead camshaft chain.

Rolling bearings are used throughout the engine. The timing side and the two center mains employ roller bearings, while the drive side is a ball bearing. The connecting rods use caged rollers at the big end, while caged needles are used for the wrist pins.  

Aluminum pistons, slightly domed with valve pockets, are of three-ring design, with two compression rings and one oil control ring.  The alloy cylinder barrel has iron liners.  Ribs are cast between the fins to reduce the mechanical noise level.  Noise suppression is also provided in the aluminum cylinder head by small white rubber discs placed alternately between the fins. These measures work quite effectively, counteracting the inherent sound transmitting nature of aluminum.

A spring loaded guide, attached to the wall of a cavity in the cylinder block, locates the cam drive chain, and takes up unnecessary slack.  Chain tension is regulated by an external adjuster found at the rear of the cylinder.  Full-length studs, originating in the top crankcase half, connect the entire head and cylinder assembly rigidly to the crankcases.

Oil pressure is provided by a gear pump driven by a steel spur gear off the crankshaft.  The pump is in the primary drive cavity, on the right side of the engine.  A double filtering process is incorporated in the lubrication cycle.  Oil, passing through a screen at the bottom of the sump, circulates through the pump, and out through yet another screen before it is fed through both ends of the crankshaft to the big ends.  Splash and oil mist lubricate the main bearings and wrist pin needles.  Oil thrown from the connecting rod big ends is thrown onto the cylinder wall for additional lubrication of the piston skirts. Camshaft and rocker arms are pressure-fed by passageways from an external oil tube at the front of the cylinder.  Oil also feeds to the critical scuffing areas of the cam lobes and rockers.  After draining down to the sump cavity, the oil begins a new cycle.   The overhead camshaft rides on four ball bearings.  These bearings are narrow, two of them at each end of the cam doubling up two narrow hearings increases load capacity over a single wide bearing of equivalent size.  

Cam profile might be described as sporting but gentle, particularly for a "single knocker." Intake opens 47 degrees btc, closes 67 degrees atc; exhaust opens 60 btc, and closes 41 atc, yielding intake duration of 294 degrees, exhaust duration of 281, and overlap of 88.  But the ramping to peak lift of .360 in. is mild.  Presumably the upper power range of this machine, now limited to a 7500-rpm redline would respond quite well to a hairier grind, and, as the 650 is obviously designed to stand higher rpm, this would be a likely place for the tuner to extract latent bhp.

On top of the head is a removable aluminum casting which carries the four individual rocker shafts; it doubles as the top half of the cambox.  When this cover is removed, access to the cam and valve train is possible.  The four valves are held on seats by a pair of inner and outer coil springs.  The spring retainers and keepers are steel.  Automotive style umbrella type rubber oil seals slip over the valve stems, and form a barrier around the end of the guide to keep excess oil from seeping down into the intake and exhaust ports.

Valve adjustment gear is reached by removing the four bolt-on triangular shaped covers at the front and rear of the cambox. Adjuster screws and lock nuts in the rockers regulate the necessary clearance.  The one-piece camshaft is hollow, with the driven sprocket in the middle and lobes for corresponding valves on either side.  Through the hollow center passes a shaft which connects the contact point actuating cam on the left with the auto-advance mechanism on the right.  Separate assemblies are found under oval covers above each spark plug hole.

Power is transmitted from the crankshaft by a straight-cut primary drive gear that engages directly with corresponding teeth on the clutch gear.  This clutch gear is connected to the clutch housing by anti-shock springs.  Needle bearings take up the clutch hub end thrust, and the multi-disc clutch transmits power smoothly and easily.

Constant-mesh rive speed transmission gears run in a common cavity with the crankshaft. The engine oil in the sump is churned up onto the gear train and lubricates the entire assembly and the ball bearings that support them. The primary drive gears are also lubricated by this method.  With one source of lubrication taking care of everything, routine maintenance is simplified.  An oil change every 500 miles or 30 days is a wise move in light of the fact that one oil is the only lubricant for all the critical wear areas in the engine.  

The rubber-mounted dual carburetors are of the constant velocity type, and throttle response is excellent.  A butterfly valve in the carburetor is actuated by the throttle cables. As this valve is opened by the twist grip, engine vacuum decreases and a diaphragm in the carburetor controls the opening of a second valve to allow passage of the fuel-air mixture in direct relation to the needs of the engine.  Efficiency and economy are primary features, as fuel consumption is determined by vacuum, not the indiscriminate yank of the throttle cable by the rider.  Air filtration is taken care of by two replaceable paper elements, which are easily reached by removal of the metal side covers.

The frame is a double loop, with a single top tube under the gas tank.  The engine unit is snugly cradled between the mild steel tubing, and ample gusseting is placed at areas of stress around the fork head arid the swinging arm pivot area. A stout tubular swinging arm contributes to the absence of undue flex and twist.

Vibration is minimal above an idle, suggesting that the frame is "well-tuned" to the engine. Stability on the road is evident at all speeds.  Steering is precise, with a slight amount of understeer in the turns.

The fork angle is suitable for a good road hike, and surface irregularities are overcome with only the slightest amount of handlebar wiggle.

Fork damping and spring rates are matched to the weight and type of road surfaces for which the machine is intended to be used. Yamaha is not trying to kid its prospective owners by calling the XS-650 a street-scrambler.  

Appearance of the new Twin is excellent, with finish of the different components above average. Chrome steel fenders set off the black frame, while the candy paint gas tank is in keeping with the current trend towards attention-getting eye appeal.  Individual tach and speedo heads, as well as the foot rests arid handlebars, are rubber mounted.  This eliminates the vibration tingle in the hands and feet that are so annoying after a few hours of riding.  Control and seating position is just right, further assuring a comfortable ride.

One or two prods of the kick starter would bring the throaty Twin to life.  Choking is necessary when the engine is cold and a two to three minute warm-up is required to allow the carburetors to respond correctly, another distinctly Japanese trait.  The sound from the twin megaphone-styled mufflers is on the loud side, which will cause the public to notice the new machine quite readily. Unfortunately, the gendarmerie as well as our cranky senior citizens will be aware of its rumbling presence, and a trip to see the local judge might result, although Yamaha states they are fully approved.

The XS-650 felt quite at home on the winding roads of the Malibu mountains, with the brakes being the only components to show signs of fatigue.  Yamaha has provided an air scoop on the front unit, and exhaust holes in the hub opposite the backing plate. Removal of the plate and plugs that cover these holes would facilitate brake cooling, and is advisable whenever heavy usage is contemplated.

In its introductory year, the XS-650 must be considered a succes fou (crazy success), having supplied all the ingredients required to please the big Twin fancier in an up-to-date, beautifully styled package.  It looks good, rides good, stays clean and shows few of the faults one would expect in a first-year model.

As delivered, it performs on a par with its peers.  The bonus:  that new engine is a racing machine in disguise.  For that reason, we fully expect that Yamaha's partial invasion of American "Class C" racing-limited previously to its rapid 250 and 350-cc road racers (and the 250 trackers)-will become complete in very short order.

Source Cycle World magazine Road Test Annual, 1970