Retrospective: Yamaha FZ750 Genesis: 1985-1986, 1988
STORY AND PHOTOS BY CLEMENT SALVADORI
It was the early 1980's when the Japanese really got their sporting act together with liquid-cooled engines, single-shock suspensions, and frames that could handle the power in the corners.
Yamaha was third in line, after Honda’s Interceptor 750 and Kawasaki’s Ninja 900, as the company put the FZ750 on the market in the spring of 1985. The 750 production class was hot back then, and Yamaha hit it right on target.
The motorcycle market and the world’s economy in general were imploding in the mid-1980s, but these designs had been in the pipeline and would not be denied. There was not much profit to be made from super-sporty models, but they were the flagships, got the attention, needed to be built…and the technology that was so expensive to come by would dribble down to more mundane bikes in future years. Every OEM was racing for the racer’s edge.
Of course the marketing folk in Hamamatsu wanted something to boast about, other than having a typical UJM in-line four with two overhead camshafts, and the engineers had provided this by adding a third intake valve to the heads. So instead of 16 valves to adjust there were now 20, but the theory was that with three valves more fuel could be sucked into the combustion chamber, and more fuel equals bigger explosion, and bigger explosion equals more power. And they stuck to that story for 20 years. Yamaha liked to call this the Genesis engine.
Nobody could deny that it was a pretty good machine, and the advertising boasted that the ’85 model had 102 horsepower at the crankshaft, and could do the quarter-mile in under 11 seconds. Slightly more real-world testing done by magazines showed 85 rear-wheel horses, and a quarter-mile in a little over 11 seconds. The public always allowed the ad guys a bit of slack, and a 15 percent reduction between crank and road-surface is acceptable. This FZ750 really was a superb show of astute engineering, and the predecessor XJ750 was an instant dinosaur. First thing to be admired on the FZ was the narrowness of the grossly oversquare engine, with 68mm bores, 51.mm strokes, hard to achieve when the cylinder liners needed coolant. The easy part was tucking the alternator and starter behind the cylinders, the innovative bit was using wet-liners only at the hottest part of the cylinders, the middle. The skinny engine was only a shade over 16 inches wide, with a narrow crank, the whole thing sitting lower in the frame and keeping the weight down where it belonged. And it was a short engine/ tranny package as well, just 6 inches from crankshaft to output shaft.
The five-valve theory was valid, considering the fact that more valves meant smaller, lighter valves, and smaller valves could be moved up and down faster and did not have to move as far to allow the mixture in. The black box cut into the fun at 11,800 rpm. For the record, Yamaha had also contemplated a seven-valve head as well, but obvious complications put that one on the shelf.
The combustion chambers were shallow in the dome, matched with a shallow dip in the pistons, and reliefs for the valves—a long time ago engine designers figured that having a valve touch a piston was not a good idea. Yamaha understood that this new-fangled unleaded gasoline could create problems, especially when running a compression ratio of over 11 to 1.
What the matching concavities did was to focus the intake of fuel from the 34mm carbs into the center, where the single spark plug was—Bang! A clean explosion was the result, with none of that dreary knocking. The carbon residues all went out a four-into-two exhaust…the astute reader will note that the original item has been changed to a four-into-one Yoshimura pipe on this 20-year-old.
The frame this sat in was a singularly unattractive but effective cradle made of box steel, with wide upper arms creating a perimeter around the top of the engine; there was not going to be any unnecessary flex in this chassis. The factory was kind enough to include several bolt-on sections in order to get the engine out. Overall wheelbase was 58.5 inches, and the steering head was angled at a steepish 25.5 degrees, both to ensure quick steering…and to concentrate engine weight onto the front wheel. To this end the cylinders of the engine were canted forward at 45 degrees, an unusually severe angle. The engine—which had no counterbalancers—was solidly bolted in, so there was the inevitable buzz at middling rpm, but since the bike was to be ridden at high rpm who should care?
A 39mm fork had a built-in brace, simple and functional air-assisted valving, and connected with the axle of a 16-inch front wheel carrying a 120/80 tire. And it was obvious that a slightly larger 140 tire could be fitted with no problem, the size that race-conscious riders preferred. This gave a rather short trail of 3.7 inches.
At the back was an aluminum swingarm with a single rising-rate shock that had adjustable spring preload and rebound damping, and an 18-inch wheel with a 130/80 tire—and a 160 size would easily go in there.
With a top speed approaching 150 mph (on a stock 750!?) good brakes were essential, so the front wheel had a pair of ventilated discs with opposed-piston calipers and the latest in semi-metallic pads. The rear disc had a single piston pushing against the disc.
The original FZ750 had a half fairing with decals denoting its five valves. Which became a full fairing on the aberrant FZ700 of 1987, destroked to 697cc, with a GENESIS decal on the tailpiece. The owner of this model has replaced several body pieces for mysterious reasons, and is lacking a couple of decals.
Time and development wait for no rider, and while the FZ700 was intended for the cost-conscious purchaser, the new ’87 FZR750R with the Deltabox frame and the way higher price was for the racer who wanted to win races. The FZ reappeared as a 750 in 1988, and then went the way of all out-moded motorcycles.
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.
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
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 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. 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 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.
Source Cycle 1975