Yamaha XS1-B

Yamaha XS1-B Road Test

Yamaha XS1-B 650cc – Motorcycle Sport July 1972
After making only 2-stroke models for the 15 years since its founding, to the point that its brand name had become synonymous with 2-strokes, Yamaha introduced its first 4-stroke engine on this model, which quickly won a popular following. The goal of building a “lightweight, slim and compact big-displacement sports model,” was achieved by mounting a vertical OHC twin engine characterized mainly by its slimness on a slim double cradle frame.

Yamaha XS1-B Road TestMaybe the British have been renowned as the builders of big vertical twins, maybe they have (or at least like to think that they have) got a stranglehold on this sector of the market. Yamaha’s answer has been to market a machine that hits right at the heart of all that is dear to the British way of motor­cycling. Naturally they have not been content merely to copy existing designs. They have introduced a valve operation by single-chain-driven overhead camshaft, electric starter, five-speed gearbox and disc front brake. None of these ideas would count as revolutionary but, as far as we can think, only Laverda incorporate them all in another motorcycle using this engine layout.

The present model, the XS650, is not Yamaha’s first venture into producing a four-stroke; two years ago they offered, for the first time, the XS1. This was basically the same as the present model except that it lacked a disc front brake, electric starter. The XS1 engine was, in fact, a scaled-down version of the Toyota 2000GT car, which was also built by Yamaha engineers. So you can see that although the Hammatsu company are famous for making very quick two-strokes they are not complete novices when it comes to producing a four-stroke.

The Yamaha is, indeed, somewhat “British” in concept. It has the same tautness that home manufacturers often give us, the same clean good looks and healthy exhaust note. It differs really only in sophistication (the Japanese being more so) and a more flamboyant approach which does not shrink from bright colours. Our machine, for example, was in orange with black and yellow stripes on the tank, and bags of polished alloy and chrome. It most definitely would not be the machine to buy if one wanted to pass unnoticed (although the exhaust note would see to that no matter what colour the bike was).

First a look at that comparative innova­tion from Yamaha: the four-stroke engine.
The overhead camshaft is driven, Honda style, by a chain which passes between the cylinders. At the top end the chain powers a sprocket on the camshaft which in turn operates the valves through tappets. Adjustment of the chain is by tensioner reached from the rear of the cylinders. The camshaft, with a bearing at each end, has the dual contact breaker assembly on the righthand side and the automatic advance and retard, operated by bob weights, on the left. The shaft is hollow with a governor rod passing through from end to end and responsible for the necessary degree of advance or retard. The whole camshaft operation is a simple one and it would be a pity if riders were to shy away from it because of the mystique that used to be attached to such a system. Honda have long since blown that sky high.

The 360° crankshaft, has four roller bearings, two at each end and two in the middle (one either side of the camshaft sprocket). On the righthand end of the shaft there is the straight-cut spur gear primary drive. Outside this gear is another smaller gear driving the trochoidal rotary oil pump. Lubricant is carried in the sump and delivered under pressure to main bearings, big end, transmission main axle, clutch bearing, shift fork guide bar and rocker arms. The crankshaft, small ends, cam chain, pistons and bore and primary gears are splash lubricated. A re-usable oil filter is located in the front of the righthand timing case with a spring-loaded ball valve in the locking bolt to act as a by-pass, should the filter become clogged. At the other end of the crankshaft is a Hitachi alternator. Twin coils are mounted under the petrol tank, and the regulator is under the seat. The 12v battery has a rating of 5.5 a/h.

The 653 c.c. engine produces a claimed 53 bhp at 7000 rpm. It has a com­pression ratio of 8.7 to 1 with a slightly oversquare dimensions of 75mm bore, 74mm stroke. One interesting piece of information to emerge from our research is that the engine unit weighs 135 Ib.

Carburettors are of the constant vacuum Solex type made by Mikuni, with a 30.6mm choke. The Japanese seem to have made a most successful job of pro­ducing this type of carburettor and the units fitted to the test machine behaved very well indeed most of the time, al­though they had a slight tendency to allow the engine to “die” unexpectedly. That was when we really appreciated the elec­tric starter! Yamaha have made a fair job ‘of adding an electric starter unobtru- sively to an existing design, for they have located it underneath the gearbox and taken the drive, rather torturously, by gear and shaft, direct to the crankshaft. Engagement is by “Bendix” gear. To turn over a powerful big twin engine, as we have often remarked in the past, re­quires quite a swing and to pass this task on to an electric starter motor producing, initially, 135 amps could give the battery a very hard life. The problem has been overcome, ingeniously, by, coupling the starter motor contact to a decompressor valve so that considerable assistance is given to the starter motor when it most needs it. It worked very well but sounded, on occasion, a trifle wheezy, especially if the motor was a little slow to fire. The logical place to mount the decompressor lever-cum-starter switch seemed, to us, to be in the traditional position under the clutch lever but Yamaha have chosen to place it below the throttle. As one also needed to operate the throttle at the same time as the starter we found it expedient to use the left hand as well.

The frame is of double cradle type with the, now, almost universal Ceraini-type front forks. The travel in these forks ought to be just over 5 in. but ours were distinctly non-standard in this respect and .at times seemed to have less movement than a set of overinflated Oleomatics! Upon returning the machine to the con­cessionaires we gathered that Yamaha were doing a little experimenting with heavier oils in the forks of the test model, and we were guinea pigs, so to speak. One pothole in central London was severe enough to knock the breath out of me. It was a situation that was not helped by very stiff rear suspension and a seat that appeared to have very little padding on “its” base even before one sat on it! One doesn’t exactly float along on a cushion of air on this machine. The seat hinges to reveal the tool tray and the battery.

Tyres on the test machine were Japanese Dunlop, 4.00 x 18in rear, 3.50 x Win front. There was a considerable amount of wet weather riding done during our period of ownership and the only time we felt any concern on roadholding was on one occasion when, with the machine banked over for a slow corner, the power came in with a bit of a rush. The machine momentarily stepped sideways but, as the power was eased, quickly settled back on to line. We were happy enough with the tyres. Brakes; disc front and drum rear. We have read other tests where the Yamaha disc has been criticized for lack of sensivity (feedback was the “in” term, which we would rather not use). We would say this was one of the best disc brakes we have ever tried—effective, effi­cient and reliable. Its only vice was a ten­dency to squeal occasionally. So full marks for the brakes.

That this machine was never really in­tended as a serious contender in this country can be seen from the handlebars. The highest, widest and possibly hand­somest bars you ever saw. Fine for town use or up to 70 mph but for real high­speed touring, agony! The instrumen­tation is clean and easy to read although the speedometer and tachometer both suffer from white light reflecting off the shell at night. The two are illuminated with a very restful green light but this effect was spoiled for us by all this ex-teraneous light. The light controls are what one might almost term “standard Japanese”. The left-hand operated dip-switch and main switch are side by side with the dip nearest the thumb. Below that the “left-to-right” flasher switch and below that the barely adequate horn’s. Yamaha clearly believe in as much illu­mination as the generator allows for they have opted for a slightly above average 27w flasher bulb. Regrettably, it is still not really good enough for daytime use. The stop light is 27w, and the tail light 8w, again slightly above average, and the headlight a whacking 50/40w. It was with­out doubt one of the best of its type we have tried and night driving behind this kind of light is a real pleasure.

The bulb is Stanley but not the more usual offset bayonet; it is in fact American pre-focus. We think that one or two owners of other large-capacity machines might well be wondering how to fit the complete unit. A parking/driving light is operated by a separate switch in the headlamp. Nor­mally we are reluctant to trust small park­ing lights for night riding but occasion­ally, around dusk, the 40w dipped beam became something of an embarrassment. The on/off switch, next to the right thumb, was all too frequently accidentally knocked to the “off” position, stopping everything. It is all very well reeling off sheets of technical specifications but the question prospective buyers are going to ask is, “Is it better than a Bonneville, and if so how?”. Perhaps we were expecting too much of the machine. We really did ex­pect the Yamaha company to have over­come the one real problem that vertical twin owners never tire of complaining about—vibration. It is not an insurmount­able problem. Norton have at least har­nessed it and the 450 Honda seemed to be well on the way to beating the bug com­pletely. Perhaps it has beaten it by now but, as one never sees one of these excel­lent machines in this country, it is difficult to tell.

Sad to say then that the Yamaha has the old familiar bogey of vibration. True it has been smoothed out in places but when one is accelerating in the gears or pushing the willing 650 along in excess of 4,000 rpm the bike does vibrate. The footrests are the first to pass on the message, then the seat of the pants and finally, when things are really buzzing, the handlebar grips. Please do not get the idea that the thing is shaking itself apart. It isn’t but it is not as smooth as a three-or four-cylinder machine. It is strange that in some respects the vibration does not show at all. The mirrors, for example (and the handlebars are not rubber mounted), never for a moment lost their sharpness of image. We have always felt that twin mirrors, mounted as they were on the 650, look a little “sissy” but there can be no faulting their effectiveness. A car 50 yards behind could be seen in both mirrors, clearly, and so could both kerbs. When one considers that Yamaha de­clined to import the little trail bike into this country on account of its exhaust noise one wonders just how noisy it was. This machine, too, is really quite raucous and must have caused a little head-scratch­ing before it was finally offered for sale over here. The silencers are of the fat cone type and appeared to play a small part in muting the exhaust note. Music to some but ammunition to our enemies. The exhaust pipes, while we are in that area, showed not a trace of blueing.

First-gear engagement was always silent and, generally speaking, the gearbox was as light and smooth as one could wish. The exception was if one was a little slow changing down when bulked by traffic. Then it showed a marked reluctance to travel quickly through, say, second and third and it was easy to be caught out in too high a gear. The reward for this was a sharp kick from the transmission, a re­minder that underneath was a potent motor that would not be abused. In other respects the engine was more than docile. If one eased the throttle back it was quite possible to burble along at 1000 rpm in top gear at just under 20 mph It would not, it is true, accelerate without assis­tance from the gearbox from that speed but that is, perhaps, asking too much.

The big virtue of an engine design such as this is its deep, biting response from low down. Torque. The Yamaha has it by the bucketful. It showed in many ways. Perhaps one would be pottering along in traffic at 35/40 mph (indicated, the speedometer was at least 10 per cent fast through the range). The chance to overtake would present itself and a flick of the throttle was sufficient to send the 650 soaring up to the 70s without a thought of using the gearbox. At the other end of the range, if one was pres­sing on just a little and was bulked at 70 mph a drop into fourth gave the machine just that little extra kick and in no time at all 90 was showing and fifth could be re-engaged. This is one of the great benefits of a 5-speed gearbox. With the normal four-speed box one is reluctant to hold third at high speed for as long as the motor is usually sounding over busy, but with the extra ratio to play with the sensible engineer will have provided a ratio just below normal top to give that little bonus. Yamaha have done just this and the result is most acceptable.

The Yamaha XS 650 is no lightweight, turning the scales at 427 Ib, but the elec­tric starter accounts for a little of this and the weight was never really noticeable the large bars giving one more than enough pull if traffic threading was in hand. At the top of the range the machine did not be­have quite as we expected it to. It coped admirably up to the legal limit and be­haved just as a well designed vertical twin should. Then, while being pushed through fast, bumpy bends, it would show a ten­dency to weave a little. We are in some­thing of a dilemma over this for we under­stand that a previous tester threw it down the road one wet day (we are not gloating, it could just as easily happen to us). Yamaha say they suspect a slight twist in the frame, citing the machine’s reluctance to steer hands off since the accident. This may well be so, and how can one criticize a feature that may not be typical of the model? Rest assured that we will pursue this theme if the opportunity arises to try another XS650.

Claimed top speed of the big Yamaha is 115+. It seems to us to be a little on the optimistic side and our suggestion is that 105 is nearer the mark. More impor­tant, in our view, is the way it reaches its maximum. There is no flattening off in acceleration until the speedometer needle is past the three-figure mark and even if one chooses to accelerate from 40 mph in top gear the magic 100 still appears in quick time. The XS650 is, without doubt, a fast roadburner but this side of its nature is let down by a 2 l/2 gallon petrol tank. We ask you, 2% gallons! If the machine is ridden really hard 45 mpg or less can be expected, giving it a range of just over 100 miles. With our new motorways that means stopping for petrol every 80 minutes or so, allowing for service station frequency and an ade­quate reserve. The exhaust note was more than a little embarrasing in town and consequently we kept a careful throttle hand. Every cloud has its silver lining, though, and under these conditions we found that the machine would return over 60 m.p.g. We would prefer to spend our petrol though rather than merely let it slowly drain away.

We find ourselves in the position of worrying that we may have been a little hard on this big ‘un from Yamaha. You see, it is as least as good as most others in its class and offers a degree of sophis­tication rare in home-produced machinery at roughly the same price (£650). Trouble is, we expected much more. We imagined that Yamaha would not have considered making a vertical twin unless it could beat the pants off long-established British rivals. Frankly it can’t. It is as quick as some, has disc brakes and electric starter and first-class lighting but it wasn’t the most comfortable machine that we have ever tested and vibrated, if anything, slightly more than the Bonniville we tried last year. Viewing it as a direct rival to the British big bike brigade, the question comes down to sophistication experience with a possible bonus going to the home industry when it comes to cost and availability of spares. The XS 650 Yamaha is a bright and cheerful addition to the market and certainly has not dis­graced itself but we cannot help feeling that Yamaha’s heart is really in the world of two-strokes. If they are going to score heavily in the big bike league it will be with the water-cooled four-cylinder two-stroke they introduced in Tokyo earlier this year.