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Yamaha XJ 750 Seca
It was at an early-season Grand Prix last year that Kenny Roberts leaned his YZR500 into a turn while still braking hard, the forks bottomed, the rear wheel a foot off the pavement and the front wheel turned slightly to the inside. Spectators gasped, photographers fired off frames and Roberts continued around the corner, exiting on two wheels and winning the race. The best photo of that impossible, magic moment was published in a German magazine. A race mechanic saw it, clipped it out and hung it on his shop's wall, where it now drew the attention of a factory-sponsored Superbike pilot.
"That's the biggest problem we've got with our big brakes," the rider said, pointing at the picture. "You can't keep the rear wheel on the ground when you're using the brakes. It's really bad when you're going into a turn with the rear wheel up in the air and the bike wants to get sideways real bad. What we need is anti-dive forks."
Anti-dive forks. What that superbike rider still wants but doesn't have, what Kenny Roberts' YZR500 lacked for most of the 1980 Grand Prix season, comes standard on a new street bike known as the Yamaha XJ750RH Seca or Seca 750 for short.
It's a curious mix, this Seca 750. The anti-dive forks of a race-bike. The shaft drive of a tourer or cruiser. The digital monitor system of something out of Star Wars.
It isn't the quickest, nor the fastest, 750-class motorcycle. It isn't the lightest, nor the best handling. But it comes close in every respect, and it wins the sales features contest hands down. It may be the most interesting motorcycle of the year.
The dchc, two-valves-per-cylinder, plain-bearing engine is derived from the XJ650 Maxim powerplant introduced in 1980. The Seca 750 displaces 748cc, with a bore and stroke of 65 by 56.4mm. Besides having a larger bore and longer stroke than the 63 x 52.4mm, 653cc Maxim engine, the Seca engine isn't rubber-mounted. There are other changes as well. The 750's pistons are shorter overall, measuring 56.1mm from top of dome to bottom of skirt, and shorter from the center of the piston pin bore to the deck as well, measuring 25.7mm. The XJ650's pistons are 59.8mm tall overall and have a deck height of 26.2mm. But the 750's pistons are actually taller from pin center to skirt bottom, 27.8mm to the 650's 27.5mm.
Piston dimensions were changed to suit the longer stroke and hold down reciprocating weight in spite of the larger bore. The 750's pistons are just .17 oz. heavier, 5.64 oz. to a Maxim piston's 5.47 oz. The 750's connecting rods are longer, but otherwise dimensionally identical to the 650's rods.
The 750's crankshaft has the same size plain shell bearings as the 650, and the crank width is identical. Crank pins are the same diameter, but are repositioned due to the larger stroke. Like the 650, the 750 has a gear primary drive, the power flowing from crankshaft to clutch hub to mainshaft to layshaft to a middle gear shaft incorporating a spring-loaded ramp-type shock absorber (or cush drive) and from there through bevel gears to the driveshaft. The layshaft is positioned underneath the mainshaft in the lower half of the crankcases, to hold down engine length, with the middle gear shaft behind the mainshaft. The system is relatively simple for a shaft-drive, transverse four-cylinder engine, but still uses one more shaft than the simplest Fours. However, because the engine uses a gear primary directly to the clutch hub, it has the same number of shafts as the chain-drive Seca 550, which uses a link-plate primary chain to drive a jackshaft, which then drives the clutch hub through a gear.
The 750 engine uses a small link-plate chain to drive an auxiliary shaft, which carries the generator behind the cylinders, thus avoiding mounting the generator on the crankshaft and reducing overall engine width. And the 750 is narrow, 18 in. from crankshaft end cover to end cover.
Like the Seca 550, the Seca 750 has a Yamaha Induction Control System (YICS) cylinder head. (YICS was explained in detail in the April, 1981 CW.) Basically, YICS consists of small, auxiliary sub-intake ports connected by a passageway in the head. A small port intersects each main intake port and feeds a high-velocity stream of air-fuel mixture into the main port during the intake cycle.
That high-velocity stream swirls the charge coming through the main port, resulting in a more dense, more evenly distributed mixture in the cylinder. The denser charge improves fuel efficiency and gas mileage, without reducing peak horsepower.
Beyond the YICS feature, the 750's cylinder head is conventional, with two valves per cylinder and dohc driven from the crank by a roller chain. The valve sizes are the same as on the Maxim 650, 33mm intake and 28mm exhaust. But valve timing is different, to match the larger displacement, the 750's intake valves opening 38" BTDC and closing 58 ' ABDC for a duration of 276°; the 750's exhaust valves opening 56 ° BBDC and closing 36 ° ATDC, for a duration of 272°. The 650's intake valves are timed at 34-58 for 272° duration and the 650's exhaust valves are timed 66-26, again for 272° duration.
The 750 also uses more intake valve lift, 8.8mm versus the 650's 8.5mm, while exhaust lift is the same in both engines at 7.8mm.
The camshafts in both engines operate the valves directly through valve cups with lash adjustment shims riding between cup and cam.
The end result of all the numbers is that the 750 makes more horsepower than the 650, a claimed 76 blip at 9000 rpm for the Seca versus a claimed 64 bhp at 9000 for the Maxim. Torque figures are up as well, from the 650's claimed 38.3 lb.-ft. to the 750's 44.9 lb.-ft., both at 7500 rpm.
The numbers are believable, at least for purposes of comparison between the related 650 and 750 engines, because the 750 feels stronger and produces better numbers at the dragstrip as well. With an elapsed time of 12.34 sec. and a 106 mph terminal speed, the Seca 750 is competitive with other 750 class machines tested at the same dragstrip by the same rider. The Seca's 119 mph top speed reached in a half-mile run is equally competitive. (For purposes of reference, the 1980 Kawasaki KZ750 ranks as the quickest 750, having turned 12.26 and 107 mph, with a half mile speed of 120 mph.)
Yamaha engineers did not devote themselves solely to making power. Suppressing noise also attracted their attention, and that's why the 750 engine has thick rubber sleeves on four of the five front cylinder studs. The fifth stud, located on the right-hand side of the engine, doesn't have a rubber sleeve because its cylinder hole is used as the pathway for oil delivered to the camshafts and valve gear.
Another move to hold down mechanical noise is the use of a two-piece clutch cover. There's the usual cast clutch cover, with another, exterior cover bolted on from the inside, with a thick rubber gasket sandwiched between the two covers. Then again, the 750 has crankcase end covers cast of thicker aluminum than those on the similar 650 engine, and of course the reusable, thick rubber cam cover gasket also contributes to reducing noise.
That cam cover gasket seals as well as it does because the bolts securing it cannot be overtightened. Each cover bolt is shouldered, and bottoms in threaded steel inserts fitted into the bolt holes located in the head casting and the cam caps.
Each cam is held by three bearing caps with what appears to be a fourth cap (on each cam, located next to the cam sprocket) actually serving only as a fastening point for the cam cover.
Considering that it has shaft drive, the Seca's road-ready weight (with half a tank of gas, mirrors and standard tool kit), of 503 lb. is remarkable. Considering that the bike comes with anti-dive and a monitor system and cast aluminum wheels, we can't help but wonder what it would weigh without the extras and with wire spoked wheels. The only 750 Four that's lighter is the chain-drive Kawasaki K.Z750, weighing 491 lb. ready to roll. The Maxim 650, which doesn't have anti-dive, computer readout and sender units, or dual front disc brakes, but which does have cast wheels, weighs 470 lb. with gas.
If heavier than the Maxim, the Seca 750 is still a very light 750. And those features!
Start with the front brakes. They are conventional enough looking dual 11.6-in. hydraulic discs. But follow the hydraulic lines up from the calipers and you'll find they end at a master cylinder tucked behind the headlight, between the fork legs. From there, a cable leads to the front brake lever. The brake system is set up that way so that Yamaha designers could avoid having a master cylinder reservoir perched on the handlebars—instead, they sought a clean, sleek, uncluttered look. The bars themselves are hidden and shaped by padded rubber covers, interesting items that prevent securing the bike into a truck or van the usual way. (Tie-down straps must be hooked on the front downtubes' cross piece, below the gas tank, instead of on the handlebars).
Then there's the anti-dive system, linked to the brakes. When the brakes are applied, pressure from the brake fluid moves a spring-loaded piston valve against a seat, restricting an oil damping passageway in the fork leg. With oil flow through the compression damping system restricted, the forks compress more slowly and travel less than usual, avoiding the rapid compression and bottoming experienced when the brakes are used hard on some bikes with conventional forks. If the Seca's front wheel hits a bump when the brakes are on, the sudden increase in damping oil pressure pushes the piston valve back off its seat, allowing the forks to compress more to deal with the bump.
And that monitor system! Sensors and switches tell the system if the sidestand is down; if brake, oil, fuel or battery fluid levels are low; if headlight or taillight bulbs are burned out. For example, if the plug-in battery level sensor (which takes the place of one cell cap) tells the system that the battery water is low, then a liquid crystal display (LCD) reading "Batt" (located between the speedometer and the electronic tach) turns on, and a big, red warning light starts flashing to catch the rider's eye. In addition to the warning light and LCD readout, leaving the side-stand down and engaging a gear (even with the clutch pulled in) will kill the engine, making it impossible to ride away with the sidestand down.
It's that flashing red light that the rider new to the Seca discovers first, after he's turned the key and hit the electric starter button. As soon as the engine starts, the LCD readouts flash on in succession, a test sequence to show that they're all present and accounted for. Then, (assuming everything else is normal), the LCD for the side-stand (reading "STND") stays on as that bright, pulsating red warning light at the top of the instrument console^ incessantly warns the rider that yes, something (the sidestand) is holding up the motorcycle as it waits for him to climb aboard. The light and warning LCD go out when the side-stand is retracted.
The engine can be reluctant to start when cold, especially if the rider turns on full choke (the choke lever is conveniently located on the left handlebar control pod, within easy thumb's reach of the left grip) and then turns the twist grip a little as well. The choke is actually an enrichening circuit in the CV carburetors, and the engine starts best with full enrichment and zero throttle application. Once running, it only takes a moment or two before the bike will pull itself down the street, with the enrichener still on. But the rider has to be careful at the first stop, because stopping hard with the enrichener on—even partially on—floods the engine, leaving the rider to curse and crank the engine with the electric starter for what seems like an eternity.
The riding position is good, comfortable enough for average height riders, but the sculptured seat grows hard after 40 minutes or an hour in the saddle. The engine, lacking rubber mounts because this is, after all, called a sport bike, is reasonably smooth for a rigid-mounted Four. But compared to some, especially those with the increasingly popular rubber mounts, the engine seems buzzy, the high-frequency vibrations felt most through the handlebars. Funny how a decade ago such an engine would have been heralded as setting a new standard in smoothness and vibration-free riding. How far we've come, so quickly!
The mirrors, stylish rectangles on angled stalks, remain clear and provide good rearward visability.
Then suddenly reflections on the feel of the bike are interrupted, the reverie on a straight road shattered, by a flashing, flashing, flashing, flashing red warning light demanding instant attention and action.
A hurried glance at the instrument console and LCD readout reveals ... not a blown engine ... not certain doom ... but the mundane fact that soon, very soon, this motorcycle will demand that the fuel pet-cock be moved to the reserve position. There's less than one-quarter tank of gas left.
The fuel gauge incorporated in the LCD panel consists of four quadrants. If
four are lit, then the fuel level is somewhere between 3/4 full and full. If
three are lit, the level is between one-half and 3/4; two lit means one-half
tank or less. When the fuel level in the 5-gal. tank gets close to the 1.0 gal.
reserve level, the last LCD level quadrant disappears, the word "FUEL" appears
in LCD letters, and the warning light shakes the rider's attention away from
whatever held it, demanding that something be done at once.
It's hard to monitor traffic when a flashing red light screams for attention, a forseeable problem. So the Yamaha system comes complete with an override button. Push it once and the flashing warning light stops flashing, becoming content to simply glow. Push it again and the glowing light is extinguished, leaving only the LCD display as a warning.
So far, so good. But as in many things, the actual application of such a fine system is not quite so precise as it is on the drawing board or on the smooth tracks of a manufacturer's test facility.
In real world use, the Yamaha warning system is too sensitive, the fuel level sensor too easy to trigger. Grab a handful of throttle at the quarter tank level. As the bike accelerates, the fuel in the tank moves back, away from the sensor, and the fuel level warning light flashes away. Grab the brakes and it goes out, the first quadrant coming on again. Hit a bump in the road and the warning light returns, flashing brilliantly for a second, disappearing just as suddenly. Lean into a turn, and somewhere near the apex the red flashes start again, continuing as the bike accelerates out of the corner.
Fail to find a gas station, or intentionally ride into the tank reserve capacity, and the light becomes more incessant. Okay, the rider should reset it. So push the button, and the flash becomes a glow, and another push puts it out. Except that sometimes, on our test bike anyway, pushing the button did nothing. And when it did actually extinguish the light, the first bump or use of the brakes or turn started it up again. One particularly rough section of interstate highway set off the system 30 times in 20 miles, the rider reaching for the override button time after time, gaining only a few tenths of a mile relief before the flashing started again.
What we have here is a good idea foiled by over-sensitive sensors, and re-calibration or a time-delay in the system is needed.
Happily, the Seca 750 gets great mileage—54.9 mpg on the Cycle World mileage loop, a mixture of city, country and highway riding—so encounters with the hyperactive low-fuel-level warning system are punctuated by long miles of warning-free riding. Test bikes are ridden at close-to-legal speeds on two laps of the 50-mi. mileage test loop, but even at extralegal speeds the Seca gets good mileage. Mixes of 85-mph blitzing and 65-70 mph cruising on open-country roads typically yielded 39-41 mpg, and the tank that included 85 miles at full throttle on a road racing track out of a total of 107 mi. still gave an average of 27 mpg.
The other instruments, the mechanical speedometer and odometer and the electronic tachometer (driven by the electronic ignition system) are easy to read. The quartz-halogen headlight is bright, splashing a broad patch of light over the road on low beam, and sending a piercing stream of light down the street on high beam. For foggy nights, there's a yellow fog lamp located under the headlight and controlled by a separate switch.
The horn button is located between the turn signal switch (the turn signals are, as usual, self-cancelling) and the choke (en-richener) lever, a position which sometimes makes it hard to find the horn button when wearing heavy cold-weather gloves.
It's during straight-line cruising that a benefit of the Seca's anti-dive forks should first show up. Although anti-dive originated as a way to keep the racebike's rear wheel on the ground during braking, its function on the Seca is different. A racebike, with a sticky, slick racing tire on a wide front wheel; huge, ultra-powerful front disc brakes; low overall weight; and a short, quick-turning wheelbase can actually bring to life the old novice street rider's nightmare of grabbing a bunch of front brake and flipping the entire bike forward over the front wheel.
For racebikes, limiting weight transfer and fork dive during braking increases control, avoiding most or all rear wheel lift.
On a street bike, with treaded tires compounded for long life; smaller, less powerful brakes; and longer wheelbases, flipping over the front wheel isn't a problem. A few riders can get the rear wheel up off the ground if they work at it on smaller, short-wheelbase bikes, but it isn't something that happens accidentally.
But a motorcycle designer still must be conscious of fork dive and weight transfer and the effect it has on handling, braking and cornering clearance. In many cases, a motorcycle will be built with fork springs that are actually a little too stiff for best small-bump compliance and comfort, but which control fork dive. In other words, absolute comfort is compromised to control dive and bottoming during braking.
Which is where the Yamaha system comes into play on the Seca 750, a street bike.
The Seca 750's anti-dive controls fork dive and avoids bottoming during braking, so the fork springs can be softer for better comfort.
The Seca 750's anti-dive system is adjustable, via a screw underneath the anti-dive fitting on each fork. Changing the adjustment changes the amount of compression damping oil restriction that takes place when the brakes are applied. The system does work, and the difference can be felt compared with a non-anti-dive bike, but the forks still compress under braking—just not as much and not as fast. Besides the anti-dive feature, the Seca's forks also have air fittings, so pressure inside the fork legs can be changed to suit riding conditions.
Set up for the street, the Seca's forks are reasonably compliant, doing a good job of dealing with the repetitive small bumps that put fork compliance to its maximum test.
The rear shocks, which have the usual adjustable spring preload and adjustable rebound damping, don't do as good a job as the forks in dealing with small jolts, such as concrete highway expansion joints. But as the test bike accumulated more miles, the rear suspension did loosen up and compliance—and rider comfort— improved. Then, set at minimum preload and minimum damping, the shocks gave a comfortable ride.
On the street the shaft drive doesn't draw attention to itself, something Yamaha engineers have worked long and hard to achieve. The effectiveness of Yamaha's efforts to eliminate rear-end rise and fall (depending upon whether the throttle is opened or closed) with the shaft drive was driven home when one test rider caught himself thinking—as he rode into a rainstorm—that he should have oiled the drive chain that morning. What the shaft drive does, and a big reason shaft drive is popular with riders, is eliminate drive train maintenance except for longinterval replacement of bevel gearbox oil.
The Seca is advertised as a high-technology sport bike. The high-technology part of the bike is obvious and clear. And that the Seca really is a sporty bike becomes clear after the first long ride.
It looks racy, swoopy. Styling is eyecatching and detailed. But styling doesn't make a true sports bike: Performance does. And the best place to sort out performance capabilities safely is at the racetrack.
The dragstrip times indicate that the Seca has good power. But it makes that power at high revs, pulling strongest between 7000 rpm and the 10,000 rpm red-line. On the street, in the city, the Seca is happy to be shifted at 5000 rpm. On the racetrack or even being run hard on a twisty road, it demands rpm at all times.
Which both contributes to its sporty feel on the road and complicates things on the road racing circuit. Sail down the straightaway at Willow Springs Raceway, grab the brakes, downshift once and dive for the apex of the first turn. In the middle of the corner, dial the throttle back on and hold it at the stops. The Seca comes out of the turn at 6000 rpm, straining against a slight uphill on the course and a headwind as well. The rider waits, the engine slowly gaining speed until it reaches 7000 rpm, when it delivers a burst of sudden acceleration to the redline.
In the meantime, between the time the Seca exits the turn at 6000 rpm and reaches the powerband at 7000 rpm, a rider on a KZ750 riding out of the same turn at the same speed has gained several bike lengths, just due to better mid-range power delivery. The Seca's powerband is well-suited to dragstrip use, where it's feasible to keep the tach near redline at all times. But on the road race course, where coming off corners hard is essential for fast lap times and high straightaway speeds, the rider can't always keep the engine on the powerband with the standard wide-ratio street transmission. Because the bike has a shaft drive, gearing can't be easily changed, so prospects for improving the situation don't look good.
It's at the racetrack, braking hard for turns, that the anti-dive really proves its worth. The biggest advantage the system offers is that of keeping the front wheel on the ground when braking hard over bumps. Without anti-dive, braking hard can compress the forks, using up all the available fork travel. When the brakes are used hard, there isn't any travel left to deal with pavement bumps, so the front tire bounces off the ground, locks, and screeches when it touches down after the bump. The rider must then release the front brake to maintain control.
The Seca 750's anti-dive keeps the forks from compressing all the way when the brakes are used. If the front wheel hits a bump during braking, the anti-dive valve comes off its seat and the forks respond normally to the bump, using the available extra travel. As a result, the front tire doesn't bounce off the ground, doesn't lock while in mid-air and screech upon landing, and the rider has better control and doesn't have to release the brakes while still coming into the corner.
But if the racetrack shows up the big functional advantage of anti-dive, it also points out the drawbacks of having braking action controlled by a lever linked by a cable to a master cylinder linked to calipers by hydraulic fluid. The brakes feel, well, a little vague, even if strong. And then there are the racetrack-obvious drawbacks of shaft drive.
As pointed out before, Yamaha engineers have done an excellent job of controlling the shaft's tendency to raise the rear end of the bike under acceleration (extending the shocks, the shaft pinion gear trying to climb the wheel ring gear) and lower the rear end when decelerating.
But controlled isn't eliminated, and it's when the Seca is ridden at the limits of tire adhesion and cornering clearance that the shaft intrudes.
The Seca's cornering clearance, even with stands installed, is excellent. The point where the footpegs start to touch and the edge of the tire tread are well matched, and there's enough room left beyond that point to accomodate a better set of tires.
But if the rider gets into a turn just a touch too hot, and finds himself hanging off and leaned all the way over with the pegs dragging and still needing a little less speed, rolling off the throttle lets the rear end sink and serious, hard parts start to drag, the results being a bike getting sideways and the rider vowing not to do that again.
And once the stock Bridgestone tires get hot (which took eight or 10 laps at Willow on a cold day), another shaft-related effect surfaces. Accelerating out of turns, the shaft's action on the rear suspension increases the tendency of the tire to slide. More correctly, the shaft's action under acceleration works against rear shock compression. The bike accelerates, the rear wheel hits a bump and can't react as quickly and as fully as it would at steady throttle, and the already-hot, already-marginal rear tire isn't held completely, firmly on the pavement, the result being a slide. Depending upon the corner, the bump and the speeds involved, that slide may be a simple stepping out of the rear end at the corner exit or a full-lock, dirt-track-sideways spectator thriller, as in the case of the apex dip in Willow's Turn Nine.
Beyond that, the Seca handles perfectly, remaining stable and steady even when run hard into sweeping, fifth-gear, full-throttle Turn Eight. In our tests, the rear shocks worked best with preload set at the third position and rebound damping also set at the third position It took one wobbly lap for everything to warm up, and then the suspension worked fine.
Yamaha has used the narrowness of the Seca's engine to position it relatively low, and that relatively low positioning results not only in a low seat height (30 in.) but also a low center of gravity. That, and the short 56.9 wheelbase makes the Seca turn quickly and change direction easily.
The Seca is Yamaha's high-tech bike. It's fast, sleek, and ever-so-advanced. It's a terrific sporting street bike, even if it isn't the best box stock road racer. It's packed with features, and functions well as a motorcycle all the same.
And it's expensive, with a retail list price of $3199. For reference, a Kawasaki KZ750B2 (standard) lists for $2899, a Kawasaki KZ750 LTD for $2999. The Seca is still less expensive than a lOOOcc-class machine, however, as illustrated by the Honda CB900F's $3495 price tag and the Kawasaki KZ1000J's $3799 list price.
Some people, of course, aren't interested in the kinds of technical trickery offered by the Seca, leaning more toward simplicity and a low price tag. For those people, there are plenty of other motorcycles to buy.
For those who want the trickest thing this side of outer space, the Seca has landed. And without a doubt, it's the most interesting two-wheeler of 1981, LCD's and flashing lights and all.
Source Cycle World June 1981