Yamaha XS 500

Road Test 1975

Yamaha's engineers are nothing if not fearless. Most of their experience has been acquired working with the two-stroke engine, which though exquisitely arcane in some respects is mechanically simple. Yet with the XS500B sports/ tourer they demonstrated a headlong willingness to embrace enormous complexity when given a four-stroke engine to design. In this device they doubled the usual' complement of camshafts and valves, tossed in a heaping scoop of counterweights, sprockets and chains, and even included a second oil pump to perform a task others leave to gravity. True, they divided their 500's displacement into only two cylinders, but the surrounding hardware makes Yamaha's medium-size twin one of the most intricately-contrived motorcycle engines ever fashioned. There in lies some of the Yamaha XS500B's virtues, and at least one major weakness.

Prior to the introduction of Honda's CB750 Four, the Triumphesque vertical twin had firmly established itself as the prime-mover for sports/touring motorcycles. Twin-cylinder engines were also made in Vee and horizontally-opposed configurations but the upright-inline layout was inherently more compact and less costly to manufacture, and its performance in terms of power, vibration level and ease of starting proved able to attract buyers in satisfactory numbers. Vertical twins had popularized British motorcycles in America, mighty Honda had successfully used the vertical twin to overrun the English position, and Yamaha hadn't done too badly with its own line of vertical twin two-stroke street bikes.

All these factors must have counted heavily back when the XS500B (introduced in 1973 as the TX500) was still in the planning stages. Honda's fours were looming large on the horizon, but their sales appeal had yet to be tested and vertical twins were still the safest bet this side of rising oil prices. So Yamaha opted for a twin instead of gambling on a four, and then began devoting its engineering energies to a little exercise in technological bet-hedging.

Honda had reduced the traditional (that is to say, British) vertical twin's vibration by moving its crankpins 180-degrees apart, giving the pistons a right-left, right-left marching order, rather than having them stroking in unison. Yamaha adopted that shake-reducing measure, and added further compensation in the form of contra-rotating weights driven from the engine's crankshaft. The XS500B balancer shaft is chained to the crank, and the chain wraps around four sprockets and passes through a welter of guides. Rotational reversing occurs because the driving sprocket on the end of the crank meshes outside the loop of chain, while the driven balancer shaft sprocket (and the two idlers) are inside the L-shaped loop. That's just one collection of chain and sprockets concealed by the engine's left-side crankcase cover; the other is to connect the electric starter-motor, which is behind the balancer shaft, with the crankshaft. Finally, packed into the same cavity, there's the rotor and two sets of windings comprising the machine's controlled-field alternator.

The engine's right-side crankcase cover houses an area no less solidly filled with busy bits of metal. You find there the usual oil-bath clutch and helical primary reduction gears, but that's only the beginning. There's more reduction gearing to drive the ignition contact-breaker camshaft, at half engine speed, and a pair of larger 2:1 spur gears to drive a sprocket, which drives a duplex chain leading upward past tensioners and idlers to engage yet more sprockets on the ends of the twin camshafts. Also, we must not neglect to mention the spur gear hidden behind the driven primary gear, which meshes with more gears to turn the tachometer cable and zips around corners—via shafts and skew gearing—to drive the engine's two oil pumps, the second of which has as its sole duty the job of gathering lubricant from hither and yon and then stuffing it down into the sump. Why not let the oil trickle down there, compelled by the usually reliable force of gravity? You'd have to ask Yamaha's engineers.

There are obvious reasons for the complexity of the XS500B's cylinderhead. It's one thing to decide in favor of a twin when the major opposition shows every indication of having made a commitment to fours; quite another to give away all hope of matching the fours' horsepower in case they are successful in the marketplace. Here again, as when providing a balancer to counter vertical twin vibration, the steps taken by Yamaha to make the 500 an equal for Honda's inline four led deeper into the thicket of complexities. When fours do display horsepower superiority it is because they have bested the twin in terms of crank speed and valve area. Yamaha apparently reasoned a lot of two-cylinder slack could be gathered in by giving the twin room for plenty of valves with big cylinder bores, and a very short stroke to permit high operating speeds. Hence the XS500B's 73mm x 59.6mm bore/stroke dimensions, and its four-valve twin-cam cylinderhead layout.

The Yamaha's cylinderhead really is a very nice piece of work. By grabbing the required half-crank-speed reduction ratio with spur gears, down in the crankcase, they avoided having to use the oversize camshaft sprockets that swell the Honda 450/500T cylinderhead casting; and placing the cams directly over the valves minimized valve train inertia. There are pivoted, curved-face followers interposed between the individual cam lobes and the paired valves each operates, with two clearance-adjustment screws threaded into lugs extending from the sides of each follower. There is an offset between the follower's working face and the adjustment screws, away from the pivot, which both increases the cam lift at the valves and places the screws out where they can be reached when the valve clearance is being set.

The XS500B engine's innovative complexity is not repeated in its chassis, which is constructed along entirely conventional lines but provides better than average results. Its frame is your standard collection of gusseted steel tubes, supporting the engine/transmission unit in a two-tube cradle, with the familiar telescopic fork up front and a swing-arm rear suspension. If there's anything unusual about any of the chassis specifications it's the steering geometry, which has the steering axis inclined only 26.5-degrees but the trail pulled back to 4.6-inches.

There have been some changes in the XS500B since it was a TX500. Originally the combination of minimal flywheel effect in the engine, excessive lash in the transmission engagement dogs and abrupt off-idle throttle response made the bike jerky, difficult and unpleasant at low speeds. And the real horsepower didn't begin to appear until the rider had at least 6000 rpm showing on the tachometer, which meant downshifting any time the Yamaha was asked to pass anything faster than a tree. Now this twin's constant-vacuum Keihin carburetors have been given different pilot jets, air jets, etc., a mixture balance tube installed downstream between the manifolds to smooth its slow-running characteristics, and the balance pipe between the two mufflers has been increased in diameter. Guess what? The XS500B is still jerky, difficult and unpleasant at low speeds, and still flat as a skinny schoolmarm's shirtfront until you get it up around 7000 rpm and climbing. It may indeed be better than the original TX500 we tested in the spring of 1973; it still needs help.

One of the XS500B's best and worst features is its transmission, which does a lot to amplify the jerk and lurch set off by those wretched CV carburetors. The transmission's five ratios are very nicely spaced, and its shift mechanism works with a crisp, light, short-throw, snick-snick-snick action you can't help but appreciate. That's the good part; the bad news is that there's enough lash in the gears and dogs to put a big hesitation in the drive train, and the lack of engine flywheel combines with the lash to create a real aggravation. You open the throttle and you'd swear the engine picks up a thousand revs before all the drive-train clearances get reeled in and everything hits the stops; chop the throttle and the same thing happens in reverse. Ride the Yamaha long enough and you'll learn to compensate for all the snatch and yank with some Quick-Finger-Maynard manipulations of clutch, brake and throttle, but if you ever get to like it, maybe you should also try a tack taped to the seat because you will have discovered a streak of masochism in your character.

Apart from the drive-train lurch, it's hard to find enough good things to say about the XS500B's behavior on the road. For one thing, this Yamaha shares with only the vastly more expensive BMW the appealing trait of having a front wheel that climbs over highway expansion strips without transmitting the motion undiminished straight up into its handlebars. The Yamaha's fork actually does telescope to accommodate such abrupt irregularities, and we are told that there has been no change in seals, dust boots or anything that would reduce static friction; the only modification from the harsher-riding TX500 has been a reduction in fork-spring preload. The fork spring-rate wasn't changed; only the amount the spring is compressed with the fork sliders in fully-extended position, or so they tell us. Whatever the details, the Yamaha 500 twin has been transformed with respect to the quality of its ride, and the transformation goes a long way toward making it a very good touring machine.

Riding comfort can be sadly influenced by engine vibration, but the XS500B scores very well in that respect, too. Its engine's inherent, crankshaft-generated rocking couple is fairly effectively compensated by the contra-rotating balancer, and you do your steady-speed cruising free from the nagging tingle and buzz characteristic of too many motorcycles. But there are times, when engine speed begins to soar, that the Yamaha "Omniphase" balancing act starts to feel more like ominous-phase. Over the 6000-9000 rpm range you'll notice points at which the vibration headache overwhelms the balancer aspirin, and you are reminded that the Yamaha does have just the two pistons flailing around down there.

Any obsessively picayune person, like a typical Cycle road-tester, can find some fault with any motorcycle—and the XS500B's seating position and seat mildly aroused our restless ire. The Yamaha twin's seat is a bit narrow for any but the slenderest of backsides, though there is some comfort compensation in the way it is padded; and its pegs are too far forward by a couple of inches. But the handlebar is just perfection, the various levers work as though they sprout directly from the mechanisms being controlled, the instruments (except for the idiot-lights) give a world of information for the price of a glance, and the pegs—albeit too far forward—are set at precisely the right height. There's a bonus down at peg-level: they fit closely against the sides of this extremely compact motorcycle, so you won't feel like you've been straddling one of Ernest and Julio Gallo's larger wine casks after a morning in the saddle. Compactness is an advantage, vertical twins are supposed to have over inline fours, and it is one that has found real expression in Yamaha's sports/touring XS500B.

Keihin's series-CV carburetors had automotive butterfly-type chokes (like their throttles, but located up at their air entry) when they first appeared on bikes like the CB450 Honda. Those now fitted on the Yamaha twin have little plunger-type mixture enrichers for cold starting and in this application they work wonderfully well. To get the XS500B awake on a chill morning you just flick the choke lever on and punch the starter button; it will instantly be running unless you try to help the engine by cracking the throttle, which will prevent the mixture enricher from doing its thing. Alas, while Yamaha's twin will start instantly it won't be ready for business until after several minutes of croupy, sullen protest. The engine wants to be warm before it does any substantial work, and it refuses to be hurried.

Yamaha's exotic, eight-valve twin can be pretty sullen about doing its job even after it is fully awake and its blood warm and circulating. Crank up a lot of throttle at anything below 6000 rpm and all you'll get in response is a noisy gargling from the carburetors. You have to get that plain-bearing crank spinning and the pistons in an up-down frenzy before the engine really comes to life, and then you've got quite a decent power band but one located unusually high on the rpm scale.

The power peak is at 8000 rpm, and the engine pulls fairly strongly from 6000 to 9000 rpm. It doesn't pull as strongly in XS500B guise as it did when carrying the designation TX500, the present drag-strip figures being 14.597 seconds and 89.73 mph compared with 13.816 seconds and 95.03 mph back in 1973, which is curious because the twin has gained only two pounds in full-tank, curbside weight in the two years.

What the XS500B does best is tear along winding roads, and Interstate-style cruising. Those things may seem contradictory, but for Yamaha's 500 twin they're really not. The bike's ride would be better if its rear suspension units had softer springs and stronger damping (which is the reverse of what it has) and the seat shape gives the feeling of being ridden out of town on a well-padded rail; those shortcomings notwithstanding, the Yamaha is a pretty good tourer. At the 5000 rpm you'll need to keep pace with speed-limit-stretching traffic out on the open road, the balancer has engine vibration so well in hand that there isn't even any blurring of the view in the mirrors, and when you're just humming along you can forget—if only temporarily—the lack of mid-range punch. Then, too, with the pegs being so close together it's easy to keep your knees in against the narrow fuel tank, and what with the bars being just right, it's a nice ride.

Winding roads are a pleasure because the XS500B really does handle exceedingly well. The bike wears a lot of rubber for its weight, the frame and suspension elements are strong enough to hold its knee-bone and ankle-bone firmly connected, and the slightly off-beat steering geometry seems to work very well with the machine's fore/aft balance. If your riding style involves plenty of squirt-stop, squirt-stop, your left foot will stay so busy people may think you're pedalling a bicycle, because the XS500B doesn't squirt worth a hoot unless you use the transmission to maintain engine speed. On the other hand, you only really need acceleration to the extent that it is necessary to slow for turns, and if the Yamaha gives away some speed on the straights it gets most of it back with cornering ability. The bike's super-stick tires and remarkably narrow head-on silhouette (nothing sticks out to drag when it's heeled over) combine with a high degree of cornering-stance stability to make it a genuine mountain-road weapon. Somehow, in that kind of riding, it feels like a lightweight despite its 458 pounds. All that mass has a bad effect on the Yamaha's acceleration, and loads its brakes to the point of perceptible fade when they're used often and hard, but it doesn't hurt the handling—which could be improved only with better rear shocks. The existing stiff-spring/ limp-damper arrangement creates a bit too much bounding around at the bike's tail to be either comfortable or entirely confidence-inspiring.

In the category of details there are aspects of the XS500B you'll love, and others you won't. The spark plugs stand right straight up from the middle of each cylinder, and while that centered location is especially good for combustion it does put the plugs right under the low-slung tank—and changing them isn't a job for the average socket and ratchet handle. Anyone with good sense will love the oil filter, which is a miniaturized automotive-type screw-on cannister that is located against the side of the transmission case, under the drive sprocket cover. Most people will be irritated by the utter lack of storage space under the locking seat, though they may appreciate the pivoted hasp for helmet D-rings that locks with the seat. Everyone who's ever worked on motorcycles held together with phillips-head screws will like the allen-head beauties used almost exclusively on the XS500B. Everyone who has had to probe around with a screwdriver trying to adjust multiple carburetors for idle will be pleased to know that one easily-reached knob handles that setting on the Yamaha. The bike's overall finish is something we can all like, as all the fits, polishing and plating have been done very neatly, but while the tank striping is a nice piece of design work it has been done with rather tacky decals, and over a paint that may best be described as a brown worthy of Browne. Finally, all those not strong enough to hold the Yamaha extended at arm's length will learn to loathe the grab handle they're expected to use in hauling the bike up on its center stand. The grab handle is located so far back along the side of the seat as to be useless in its intended purpose.

In all, the Yamaha XS500B is a package made up of numerous small to middling strengths, encumbered by fewer but larger weaknesses. It is smooth, and it does handle. Yamah's twin also is mildly overweight, and if its power band is broad and sufficiently substantial for most purposes it also begins too high on the rpm scale to be convenient. It is remarkably compact for a 500, and experience will tell you that's a virtue not to be scorned. The machine is blighted by its hair-trigger carburetors (Keihin's CVs have imparted equally unlovely low-speed manners to many another Japanese-made motorcycle) and by all the lost motion in its drive system. Otherwise, if we forswear invidious comparisons with the Honda CB550, Yamaha's XS500B comes near —but only near—being a persuasive argument for the design concept it represents. With less abrupt throttle response, a tighter transmission, and an added dollop of displacement to give it low-end punch equal to its own weight, the bike could be a real winner. As it now stands, its less attractive traits mock the XS500B engine's ornate complexities, and the first two letters in its name begin to seem like a fair description of what it is.