Yamaha XS 400R Seca

Yamaha's RD400 departed the production line in 1979 and left behind a void. The bike had been something special. In a class otherwise composed of straitlaced commuter bikes, the RD400, like the RD350 before it, was a shooting star. The other 400cc street bikes four-stroke twins, from Honda, Kawasaki, Suzuki, and Yamaha itself  were dependable, thrifty, predictable commuters, content to please practical riders who droned to work or cruised on weekends. But the high-strung RD400, a two-stroke twin, won races, embarrassed bigger bikes on twisty roads, and captivated sporting riders by proving that there was life under 500cc. It was a special bike, and it left a hole that couldn't be filled by the aging single-overhead-camshaft XS400.

The old XS400 was just that: old. It vibrated liberally, didn't handle exceptionally well, and didn't go fast at all. Anyone looking for an entry-level machine that lent itself to an occasional brisk ride would do better to look at Honda's sporty six-valve Hawk or Suzuki's GS450. But in the last couple of years, Yamaha has been on the gas, building sophisticated sporting bikes like the Se-ca 550, 650, and 750. The technology that went into those machines has finally trickled down; waiting at the entrance of Sportbikeland is the XS400R Seca.

It is built with the latest Yamaha know-how, and about the only parts it (and its Maxim-styled sibling, which uses the same basic engine and chassis) shares with the old XS400 are the grips. At last, the hole created by the governmental exhaust sniffers has been filled; the RD is back as a four-stroke.

Surrounding the new 400's DOHC mill is a redesigned chassis, featuring single-shock Monocross suspension similar to the Vision's, and a frame that uses the engine as a stressed member. The Se-ca's styling is bold, with crisp, sharp lines like the Seca Turbo. The shape of the fuel tank belies the fact that it holds a whopping 5.3 gallons and will carry a rider some 271 miles before running dry.

But who would want to spend 271 miles in the saddle of a little 400, you ask? We would.

The 400 Seca is perhaps the most comfortable machine of its size, and it's more comfortable than many larger bikes. The seating position is almost identical to the roomy Vision: slightly rearset pegs, dual-density foam seat, and a low handlebar that balances your weight against the wind at highway speeds. On the little Seca we would gladly venture anywhere we would normally only go on a larger bike.

A gear-driven counterbalancer in the new 399cc engine also contributes to rider comfort by quelling the rocking-couple vibration caused by the engine's crankpins being 180 degrees apart. The balancer rides in plain bearings and spins just in front of the one-piece crankshaft, which also spins on plain bearings.

Some vibration still reaches the handlebar and footpegs in the form of a high-frequency buzzing, but is hardly noticeable and happens only when the tach needle swings past 7000 rpm. In sixth gear, 7000 rpm is well beyond the legal speed limit, and at 60 mph the engine is loping along at about 6000 rpm.

To keep the vertical twin as narrow as possible, Yamaha employed a jackshaft system similar to the four-cylinder Seca and Maxim 550 and 750. The alternator hangs on the left end of the shaft, and the starter clutch hangs on the right. A link-plate chain drives the jackshaft off the center of the crankshaft, just to the left of the cam-chain drive. The starter-motor gear drives the starter clutch assembly, which in turn spins the jackshaft and gets the crankshaft going when you hit the starter button. The right end of the crankshaft drives the clutch through straight-cut gears, which drives the tro-choidal oil pump. The ignition is triggered from the crankshaft's left end.

Unlike the Maxim 400, which has only five speeds, the XS400R employs a six-speed gearbox. By adding an overdrive, Yamaha was able to lower the Seca's final-drive gearing and effectively close the gap between gears without losing top speed. It certainly would have been less bother for Yamaha to use the same transmission in both bikes, and the fact that they didn't shows how serious they are about the 400 class.

With the six-speed transmission, it's easy to keep the Seca screaming along in the strongest part of the powerband, which comes along at about 7000 rpm. Below that, the 400R is not dead, but it's not as lively as Honda's CM450C or Suzuki's GS450TX. In top-gear roll-on tests, performed by whacking the throttle open 200 yards from the dragstrip's speed trap, the Seca could manage only 64.6 mph. The Suzuki ripped through the lights at 69.1 mph, and the Honda posted a 67.0-mph speed.

But thanks to the close spacing of the cogs, the Seca doesn't feel that slow, at least not when you're booting the shift lever. Only when you attempt to pass a row of cars without downshifting does the lack of midrange punch rear its lethargic head. If you make a habit of keeping the tach needle hovering around 7000 rpm, the engine responds smartly.

Considering its 50cc displacement disadvantage compared to the CM450 and GS450, the Seca 400R gives an impressive quarter-mile performance. It stopped the clocks in 14.18 seconds at 91.50 mph, a time within one-tenth of the Honda and Suzuki. Terminal speed of the Seca was slightly faster than both. This indicates that the Seca is pumping out at least as much horsepower as the CM450 and GS450. Its performance also stomps the belt-driven Kawasaki 440 by half a second and nearly four mph.

Much to our surprise, the XS400R has no trouble pulling redline in top gear, which gives a top speed close to-100 mph. Not too many entry-level econo bikes can make such a claim, and there are plenty of big bikes that will never see redline in top gear.

Credit for the Seca's high-revving power goes to the design of the top end. The twin cams are a bit more radical in lift and duration than the old single cam, and the larger valves (two per cylinder) flow more fuel. Though the pistons remain much the same as last year's sin-gle-cammers and still stroke a 69.0mm bore, compression was bumped to 9.7:1 from last year's 9.3:1 by using reshaped combustion chambers. However, the pistons now stroke 53.4mm, compared to last year's 52.4mm, due to a new crank. Displacement is 399cc; last year's was 391 cc. Valve adjustment is accomplished through shims on top of buckets instead of the old screw-type adjusters. A self-adjusting, ratchet-type tensioner now ensures proper cam chain control.

Yamaha also credits much of the Seca's top-end urge to the YICS (Yamaha Induction Control System). The object of the YICS is to create a blast of incoming fuel mixture that sets up a swirl within the combustion chamber to promote more efficient burning of the fuel. In the four-cylinder Secas, this was accomplished with interconnected sub-intake ports that tap into the main ports just above the intake valve seat. The subport enters the main port at an angle, which directs the blast around the walls of the cylinder (picture a commode flushing). When one cylinder is on the intake stroke, it draws fuel mixture from the carbs of the other three cylinders, which are on the other strokes, through the intersecting subports. Since the subport is so much smaller than the main port, the vacuum created within the cylinder draws mixture from the subport at a much faster rate than from the main port. This creates the blast effect.

Since the 400 Seca is a twin with much less sub-intake area than in the wider four-cylinder engines, Yamaha cast two chambers into the cylinder block as part of the sub-intake ports to create extra volume. This allows the system to fill with as much mixture as possible from the 34mm Mikuni CV carbs before the mixture is sucked through the tiny subports during intake. With the four-cylinder YICS, the mixture travels at approximately four times the normal intake velocity. The twin's YiCS, however, shoots a jet stream of mixture into the cylinder at 10 times the normal rate.

Yamaha also claims that the Seca's YICS has increased fuel economy by 10 percent over a non-YICS equipped 400R. But while the Seca's mileage (51.3 mpg) is good, it is certainly nothing spectacular compared to the other bikes in the class.

The Seca's chassis was designed to utilize the engine as a stressed member and accommodate the single-shock rear suspension. Virtually nothing on today's 400 Seca chassis remains the same as on the old 400's, from the tip of the R's 3.00S18 Dunlop Gold Seal front tire to the 4.10S18 Dunlop rear tread.

Steering geometry has also changed dramatically from last year's 400. Rake was steepened 1.5 degrees to 26, and trail was stretched from 3.43 inches to 3.75. Wheelbase was pulled in to 53.9 inches. These changes, along with a redistribution of weight and a slightly lower center of gravity, have made the 400R a quick handler. Despite the lack of leverage in the Seca's narrow handlebar, the mere thought of counter steering is almost enough to make the Seca change directions. Yet once it begins to lean, it doesn't have a tendency to keep going, or "fall" into a turn, as some quick-steering bikes do. Like the Vision, the overall handling reminds us of the RD400: light, responsive, and even more accurate. Compared with the Vision, which has similar steering geometry, the Seca is some 60 pounds lighter (413 pounds wet) and has a 4.8-inch shorter wheel-base. This makes it a blast to flick around in traffic.

Unfortunately, the Seca's suspension also parallels the Vision's in that both ends are very soft and there is little available adjustment. The fork bottoms more often than we would like, and the front end twists a bit during braking. But the compression and rebound damping rates are close, and the front end provides a smooth, stiction-free ride over freeway-slab joints and moderate bumps. We would like to see a little stiffer springing up front or, better yet, an adjustment for air pressure so the fork could be dialed in to suit varying rider weights and riding conditions.

The rear suspension's single shock has no damping adjustment and could use more rebound and compression resistance. Like the front suspension, the ride is plush, but that is mostly due to a very soft spring. Even with the ramp-type preload collar set at the stiffest of its five positions, the shock bottoms easily on large bumps. On the softest setting, a 150-pound rider can use up most of the 3.8 inches of suspension travel by just sitting on the bike.

The fact that the Seca handles as well as it does despite the limp suspension is a credit to the sturdy frame. Even in fast, bumpy sweepers, when the suspension bottoms, the Seca never wiggles, wallows, or threatens to do anything vicious. Loading down the bike with traveling gear for three days and then blasting up twisty Highway One didn't sour the Seca's handling either.

In the tight stuff, the Seca really shines. It has an abundance of Ground Clearance, more than its Dunlop tires allow you to use. Even during full berserk rides, our most lunatic staffers could only touch the centerstand tang to the ground. We are ashamed to admit that the pegs remain virginal to this day. And the Dunlop tires work well enough to let you get the most out of the Seca's engine. On the California freeways, however, the Seca tends to follow the rain grooves rather devotedly.

Even though the 400R is equipped with only a single disc up front and a drum brake in the rear, there is plenty of stopping power at fully crazed speeds. The binders are similar to those fitted to the Vision, but with the Seca's lighter weight, they are far more effective. Again, the only complaint about braking centers around the fork assembly. Like the Vision, the Seca's 35mm fork stanchions and sliders have little overlap. This causes the front end to flex noticeably at slower speeds, but less at faster speeds, where the fork compresses more and overlap is increased.

The nicest thing about the Seca is its unitary feel. All the components work well together. The seating position is comfortable on the highway and also allows the rider to move freely during cornering. All the controls are conveniently situated, including a handlebar-mounted choke lever. The clutch, brake, and shift levers require only a light touch to operate. The gearbox complements the engine's power characteristics and contributes to the Seca's low-rpm Dr. Jekyll and high-revving Mr. Hyde personality.

The void left by the demise of the sport rider's beloved RD400 has largely been filled by the littlest Seca. It weighs about the same, handles just as quickly, has more Ground Clearance, and is faster than the red-and-white Daytona Special RD400. It also gets better mileage, has lots more range, and is more comfortable.

But the Seca 400R does more than just complete Yamaha's lineup. There are other bikes in the middleweight class that are faster, and there are others that handle better, but few combine handling, comfort, power, and economy the way the Seca does. As an entry-level machine, it is an excellent choice. As a motorcycle in The Great Scheme of Things, it is far more fun than its practical performance and $1999 price tag would lead you to believe.

Source MOTORCYCI 1ST  1992