This article originally appeared in "Cycle" magazine, July 1976 issue
Deep inside its several basic-black cases and compartments the new Yamaha XS750D is a technoid’s delight, especially with respect to its absolutely labyrinthine power train and clever self-cancelling
turn indicators. We’ll get to the mechanical minutiae in due course, but for most of you none of it is as important as simply knowing that Yamaha’s 750 triple genuinely is an all-new design and not an XS500 twin with an extra cylinder and shaft drive, as some people have supposed; that Yamaha has, with the triple, made a quantum leap ahead of other Japanese big bikes in the area of ride quality; that the XS750 is an
exceptionally nice motorcycle by any standard and at any price: and that its solid virtues and worth-while features do more to justify the triple’s price than anyone would expect in today’s inflated marketplace. Yamaha’s last 750, the ill-fated tapioca TX750, was an internal-combustion Custer’s Last Stand; the XS750 is everything the old twin was not, is many things the XS500 only tries to be, and it’s a winner to
the core.
Nobody will have to ride an XS750 more than half a day to discover the most outstanding of its many virtues, and the experienced rider will find himself in for some real surprises. A big one will be the XS750’s smoothness, which is not what we expect in large displacement triples. The late Triumph Trident’s engine was a 750cc triple, and a chassis-agitator to the end. Kawasaki’s H2 two-stroker also has three crankpins spaced at 120-degree intervals and will give you a vibro-massage you’ll not soon forget. And the Laverda 1000, with one-up, two-down crankpins, can be considered smooth only relative to bikes like Triumph and Kawasaki. So the Yamaha triple really should shake, but doesn’t-at least not at any substantial level even in its worst moments, and it’s GL1000-smooth at certain engine speeds. Most significantly, the new Yamaha is smoother than either the Honda 750-4 or the Kawasaki 903 at normal highway cruising speeds.
Another big surprise, to us, was the Yamaha’s ride. Like all those who’ve been in motorcycling for more than a couple of years we had come to assume that BMW had a hammerlock on suspension technology and that Japan’s motorcycles-what-ever else might be said for them-would forever ride as though they had solid brass spacers instead of springs. Let the record show that Yamaha swept away that quaint notion, because-give or take the smallest of margins-the new XS750 fits squarely within the same ride-quality brackets as the latest BMWs, which previously were in a class by themselves, and comfortably ahead of any-thing else (except the RD-400C) you can buy for less than $3000. Run the Yamaha over washboard ripples, potholes or even those jolting, sharply-raised expansion strips and you get a wonderfully supple response from the bike’s suspension.
For what it’s worth, the Yamaha triple has a very mellow sound. The bike’s three double-wall exhaust pipes feed into a muffler that’s more than 30 inches in length, 4½ inches in diameter and baffled like a Minotaur’s maze, so not much sound emerges from its outlet. But the subdued beat that does escape the muffler’s interior mixes with the throaty basso from the air Intake for a nice effect at low speeds, under light throttle, you hear the whine of gears and the clicking of cam followers fairly clearly, these rather harsh mechanical noises go away once the butterflies rotate open and the revs begin to build.
Yamaha claims a 4.5-gallon fuel capacity for the XS750, and you probably could get that much gasoline out of the tank by wringing it like a washcloth. Be that as it may, the capacity that counts on a day-in, day-out basis is the number of miles you can ride before switching a bike to reserve, and the Yamaha triple does provide quite a useful main-supply cruising range. One of our test riders managed to make the XS750 squander fuel at the rate of 36 mpg, but 40 mpg is a much more representative mileage figure for moderately hard riding. Surprisingly, the triple’s fuel consumption rate varied only slightly whether it was being used for freeway travel, plodding through clots of city traffic or charging mountain roads. Under most conditions it delivered a constant 40 mpg, and could be relied upon to gasp out a fuel-starvation signal just as the last of the fourth main-supply gallon dribbled down to the carburetors and the resettable trip-meter flashed 160 miles. At that point we’d flip the fuel petcocks to their reserve position and start looking for a service station, though there was-in theory-a half gallon of gasoline and 20 miles of motion left in the Yamaha.
Filling the XS750’s fuel tank is the only service it requires at less than
l,000-mile intervals. The owner’s manual says that’s how often the air filter should be cleaned, and the battery, brake fluid and tire pressure checked. You’re supposed to clean the spark plugs and check the ignition timing at
2,000-mile intervals, and the manual also suggests cleaning the fuel petcocks’
strainers and the traps at that point-perhaps because the engine will run happily on the cheapest kind of gasoline and they figure some XS750 owners will be steady patrons of the worst cut rate Petro-Swill service stations in the country.
Cam chain adjustments are scheduled 3,000 miles apart, as are straight forward replacements of engine oil and the oil filter; draining and refilling the drive system’s cavities is expected
at 6,000 miles for this last job, the bike’s tool kit contains a double-end dip stick, which is stabbed down
into the appropriate filler holes to gauge the level of the gear oil.
One item unaccountably missing from our test bike’s tool kit was a spanner for cranking the rear suspension’s lower spring collars up and down. These collars have the usual notched spiral ramps for spring pre-load adjustments, and a five-notch load range, but that doesn’t help much unless you have the proper tool. Still, with that single exception the kit was fairly complete and a very close fit inside its underseat compartment. You won’t just dump the wrenches, screw driver and pliers back in the bag and drop the whole thing into the little storage tray; some of the wrenches have to be nested inside the kit’s tubular sockets and the rest carefully compacted before the flap can be snapped closed and the bag stowed tidily enough so the seat will drop down and latch.
When we were fishing around in the kit looking for the proper spring adjusting tool we’d have settled for an improper tool, because the collars were full down and so was the Yamaha’s cornering attitude. You really are going to like what Yamaha has done for the XS750’s ride; yield to your sporting instincts and you may wonder if the admittedly superb ride has not been purchased at too great a price. The bike’s very soft springs (28/33.5 pound-inch and 106/145 lb-in progressives front and rear, respectively) and considerable all-up weight (717 lbs.) cause the chassis to sink on the suspension under cornering loads, and you’ll find the Yamaha running out of cornering clearance long before it runs out of tire adhesion in right-hand turns. You can hook the XS750 into left-handers very smartly-we wore the Honda-style "That’s Far Enough" feeler nub right off the left footpeg end-but you won't get tilted very far to the right before the exhaust system starts graunching against the pavement.
With some bikes it’s just as well there’s a built-in exuberance limiter, as they’d go into a mad wobble and pitch you into the nearest hedge if their low-hanging pipes would allow fast cornering. That’s not the case with the Yamaha XS750: the bike rocks and surges on its soft suspension when it’s ridden immoderately, and that does take a little of the precision out of
its
steering, but it never does anything bad. And the steering geometry has been very carefully arranged for the full spectrum of riding conditions, as the bikes handling is comfortably light when you’re maneuvering around at a walk, and it’s not too light at high speeds. We found only two things the XS750’s handling couldn’t handle: the bike does
lurch smartly when its exhaust system bangs down hard, and the ribbed-tread
front tire tends to get the steering twitchy when it tries to cope with freeway rain grooves.
Owners will find that the XS750’s cornering clearance can be increased by adjusting its suspension. Cranking the rear spring preload adjusters around to their highest notches will lift the bike’s tail about 14mm (.55-inch), and there is a similar upward adjustment available in the front fork. The fork tubes’ reach was established in prototype testing, and a bit of length added in the production version-which extends up through the top fork bridge, as is done In Yamaha’s off-road machines. You can use the extra length to fine-tune the bike’s steering geometry, but we think the best results will be obtained by going for a lift at both ends of the chassis and an increase in overall ride-height. Switching the ribbed front tire for one with a block-pattern tread probably would stop the twitching over rain grooves; this might have some bad effects on other aspects of the XS750’s handling, and the side-shuffle, though disquieting really is nothing to worry about.
Motorcycles aren’t necessarily comfortable just because they ride well; a hard, high-crowned seat or poorly arranged riding position can be instruments of torture. Here, the Yamaha is a mixed bag: the seat is as broad and soft as anything you could want, and if you’re tall enough its near-perfect; the short rider will find that the seat’s just a tad too wide at its forward end, and creates a pressure point at the inside of each thigh. And the footpegs are slightly too low and set too far forward. We should point out that most bikes are much worse in this regard, and have far less comfortable seats than the Yamaha. Rarely, too, do motorcycles have a handlebar as nicely angled and placed as the one on the XS750. The Yamaha gives you a very good riding position: with the pegs up one inch and back two, it would be nearly perfect. Moving the pegs would also move your ankles away from the drive transfer case and swing-arm pivot housings-which became (literally) sore points for one of our test riders.
All of us became accustomed to the triple’s abrupt off-idle throttle response and drive train lash; we never learned to like it. The constant-vacuum Mikuni carburetor has the advantage of compensating for ham-fisted riding techniques, as the slide automatically adjusts its position over the fuel nozzle to keep the air speed high even when the butterfly is suddenly snapped open. This automatic compensating feature gives exceptionally good mid-range throttle response. But it also combines with the butterfly-type throttle valving to make for a too-rapid transition from idle to serious power; it’s just as though the first degree of twist-grip rotation flips the throttles from closed to an eighth-open.
As noted, the XS750 is afflicted with that most familiar of ailments-drive train lash. There is, by careful measurement, a loose 18-degrees of free motion in the bike’s rear wheel with the transmission in first gear, which probably is less than the average for Japan’s touring bikes, but still too much. A contact at Yamaha said the between-faces clearances at the shifting dog teeth (where the lash originates) could not be closed any farther without introducing an engagement problem. If we may be presumptuous and offer advice, we would guess that they’ve made the shifting-cam drum diameter too small, which results in the shifter-fork camming slots being too steep-and this is reflected in a somewhat notchy feel at the shift lever, an insufficiently positive force to drive the shift dogs into engagement, and a tendency for the dogs to bounce apart instead of meshing unless there is a very considerable gap between their teeth. They’ve opted to save space inside the transmission with a small shift drum, compensated for the engagement difficulties with dog-teeth clearance, and created an objectionable degree of drive-train lash.
In every aspect other than that mentioned above the Yamaha triple’s power train is a marvel, and one that appears to have been arranged with a sharp eye for results and a blind one for cost. Some form of chain-and-sprocket primary drive had to be used to carry power from the crank to the clutch, as even the slight clearance between the crankshafts main journals and their sup-porting plain bearings would upset the meshing of gear teeth. Yamaha wisely chose a Hy-Vo chain, which has proven to be both silent and absolutely reliable in such applications. Moreover, rather than accept an overhung load on the clutch, they provided an extra support bearing on that assembly’s outboard side.
The XS750’s clutch hub contains the first of its two drive-shock cushions. In this one, the drive goes through a collar with three V- shaped cam slots and three cylindrical pins before passing into the gearbox. Belville spring washers force the pins down into the points of the V-slots; drive shocks move the pins up the sides of the slots and are softened by the resulting compression of
the spring washers. There is a second, simpler, shock cushion (a pair of two-lobe cam rings held together with a coil spring) in the cross-shaft that connects the gearbox output shaft with the right-angle gears that turn the drive 90 degrees for its trip back to the rear wheel. The spur gears that transfer the drive out of the gearbox to the cross-shaft, presently having 34 and 32 teeth, respectively, can easily be changed to
get a different overall drive ratio. Alternative gear sets are not presently available, but-according to our sources at Yamaha-are scheduled for the not-too-distant future.
Right where most motorcycles have a transmission output sprocket, the XS750 has a bolt-on casing containing a pair of spiral-bevel gears. The drive gear has 19 teeth and the driven gear 18 teeth, because hunting tooth gear sets are less noisy and less apt to be troublesome than pairs with an equal number of teeth. And the driven gear feeds drive torque to a very tricky universal joint placed right at the swing-arm’s pivot line. Conventional U-joints won’t transmit rotation at an angle without setting up cyclical speed variations, which are reflected as torque pulsations in the drive system. Yamaha, unlike BMW or Honda, bit the financial bullet in this matter and went to an expensive constant-velocity U-joint-one in which the drive passes across six large ball bearings carried in grooved inner and outer spherical members. This kind of U-joint will operate at large angles without any change in input/output speeds and without creating any drive pulsations, which is why it is widely used in front-wheel-drive automobiles. Ordinary Hooke joints are cheaper; they aren’t as good.
The penultimate step in the drive’s circuitous trip from the crankshaft to the rear wheel is along a drive shaft inside the left swing-arm tube. This drive shaft is only 17mm (0.67-inch) in diameter, and doubles as a torsion bar drive cushion. The shaft’s small end is splined into the permanently-lubricated, sealed U-joint; its aft end is flared, with spherical-segment splines, and slips inside a splined collar on the final-drive pinion shaft. The last link in the drive train is an automotive-looking ring and pinion assembly in a light-alloy housing (32 teeth on the ring gear; 11 on the pinion) and is unusual only for having a straddle-mounted pinion gear. That is to say, in addition to the usual tapered-roller support bearing behind the pinion gear, there is a needle-roller bearing holding a small-diameter extension on the pinion gear’s nose.
In all likelihood XS750 owners will not have occasion to see much of the bike’s elaborate drive train scattered around a service-department workbench. Similar systems in automobiles outlast all other components. The important thing for owners to know is that Yamaha has made removing the rear wheel for flat-fixing a fairly straight-forward, easy task. Remove two bolts and you can swivel the rear fender extension up out of the way; two more and you have the muffler heat shield off, which clears the way for pulling the axle. The axle is held by a large nut at the drive-casing end and by a small pinch bolt on the right side, and when you pull it out of the wheel hub a spacer will drop out-creating enough side clearance to let the hub move off its drive splines and then straight back away from the swing arm. The rear brake disc stays bolted to the wheel; the brake caliper is left in place on its bracket.
XS750 owners obviously will not have to give any time to wheel maintenance, beyond an occasional cleaning, because the wheels are very sturdy one-piece aluminum castings. No spokes to tighten; no wobble develops in the rims. The front wheel is a 19-incher with a 1.85 inch rim width; the rear is a 2.15 x 18, and neither is intended to be fitted with tubeless tires.
We’ve said, on many occasions, how well we liked Yamaha’s fixed, twin-piston brake calipers, and used them as an example pointing to the fundamental advantages of that layout. But the new triple has single-piston calipers on floating mounts, and these work extremely well. If you’ve had experience with other dual-disc, floating caliper front brakes you’d expect that the XS750’s would be powerful but with a spongy feel. In fact, the triple’s front brake is powerful-and it’s both solid and entirely predictable. You couldn’t want anything much better. And the rear disc also works well, with a good ratio of pedal-pressure to braking action and no surprises.
Another point we’ve made much of is that front brake calipers should be mounted behind fork legs, which brings their mass close to the steering axis and reduces the pendulum effect of the fork assembly. The Yamaha XS750 has rear-mounted calipers, and we’d like to think it gains a little steering precision thereby, but that’s not the most remarkable aspect of the fork layout. What we found really amazing was that Yamaha had found a way to virtually banish static friction from the fork sliders, and this accounts-in large measure-for the triple’s ultra-smooth ride. Yamaha’s answer to the persistent fork "stiction" problem was to provide a slippery plastic lining around the top of the fork slider, where the load is high and the lubrication scanty. The low-friction lining alone isn’t the whole story: they also installed very soft, two-rate springs, with lots of preload. So you don’t get a lot of friction feeding road shocks up into the handlebar, and the first couple of inches of fork travel-which will absorb 99 percent of the bumps in most roads-is supported by a pair of very soft 28 lb-in springs.
Yamaha’s cleverness was not exhausted with the triple’s front fork. The bike also has the company’s new self-cancelling turn indicator system, which turns off after 10 seconds, or after the bike has traveled 150 meters (164 yards), depending on which occurs last. At freeway speeds you cover 150 meters in a twinkling, so the flashers keep going for 10 seconds; you’ll wait more than a minute for some traffic lights, and in that case the indicators will keep on indicating until you’ve made your turn and gone the 150 meters. It’s all handled by a little black box that senses distance by counting the pulses coming from a magnetic reed switch in the speedometer head, and has its own 10-second clock as part of an integrated circuit board. Or, if you’d rather cancel the signal yourself, punching the turn button in with your thumb does the trick.
The XS750’s engine isn’t particularly clever, but it strikes us as being completely sound. In this design, Yamaha forgot about multi-stage camshaft drives and contra-rotating balancers, and embraced a perfectly straightforward plan of action. The crank is a one-piece forging carried in plain automotive-type bearings, and the two-piece forged connecting rods have plain inserts to run
against the crankpins (the rod’s small ends are copper-plated instead of having pressed-in bushings). The crank’s right-hand end is the power takeoff; the left end drives the cams and the oil pump. A single-stage chain drive turns the cams, which work the valves, two per cylinder, through inverted-bucket followers-like an Offy, or a Z-l-and valve clearances are set by substituting various thicknesses
of follower face
discs, exactly as in Kawasaki’s big four. Just outboard from the camshaft drive sprocket on the crank extension is a spur gear that meshes with another on a cross shaft, and this gear-set is the first stage of the oil pump drive; it also is driven, through a one-way clutch, by the starter motor. And outside the sprocket and gears is a tang that drives the ignition’s contact breaker cam. Nowhere is there two parts where one
could be made to do the job, and that’s a change from some of the Yamahas of the recent past.
The triple probably could be made to pump out a lot of power, and a factoring of test weight and drag-strip performance says it’s no weakling in its present form. But the engine’s nearly-flat-top pistons, willingness to run on almost any fuel, and 276-degrees duration valve timing (with 72-degrees of exhaust-intake overlap) say loudly that the engineering emphasis has been on tractability and long-term reliability. Somebody is sure to start making muscle accessories for the triple before the first hundred leave the dealer’s showrooms; Yamaha has decided not to try butting heads with the mighty Z-l, and has pulled just enough power out of the triple to let the bike hold its head up when in the company of other big bikes.
Straight-line performance isn’t the XS750’s forte, which that won’t matter to anyone who understands that there is such a thing as enough. And the triple is fast enough to meet any reasonable requirement for passing or zinging up mountain grades. In any case, its strong appeal isn’t in the realm of muscle, but in nicety and finesse. The three-cylinder engine’s vibrations, if any, get filtered out by well-engineered mountings and rubber-bushed handlebar clamps. Yamaha has blessed the bike with the smoothest ride this side of a BMW, and it handles more than acceptably within the limits imposed by cornering clearance. It’s a good-looking machine, with an air of quality about it that doesn’t fade when you subject individual components to close scrutiny. And it’s an awful lot of motorcycle for the money. But more than anything else, what-ever price Yamaha asks for it, the XS750D is simply very, very nice; a pleasure to ride anywhere, and for any reasonable time.