Kawasaki GPz 550

The Uni-Trak CPz550 really set the middleweight class alight when it came out in 1982. It was the result of steady development that produced a mid-weight that could give the big boys a good run for their money.

They had it pretty much their own way since Honda's CBX550 wasn't out and Suzuki admirable 550 wasn't out and Suzuki's admirable 550 was not aimed at such a sports market.

Obviously the GPZ600R has largely replaced it, but the 550 still has plenty of fans who prefer its handling, lines and air-cooled integrity.

Power went up from 54 to a claimed 65bhp, so the move to monoshock was a sensible and successful one. A steady stream of improvements in every department shows what can be done when the manufacturer sticks with one design and refines it rather than chucking it all away and starting with a new configuration.

Like a lot of the classics here, it is a blend of many attributes that makes it so good just as much as outright performance.

The GPz evolved from the 500 in 1980, becoming the Z550A1. The basic engine configuration remained the same throughout with double overhead camshafts running off a central chain.

58 x 52.4mm bore and stroke, producing 54bhp at 8500rpm. This model had seven-spoke cast wheels and a slab-sided tank, with black engine and chrome exhaust

A Z550C custom version did appear but we'd rather not talk about it

In '81 they got closer with the Z550GP, still with twin shocks but with 58bhp at 9000rpm, a handlebar fairing and an oil cooler. The engine and exhaust were painted in black, but it usually fell off when you switched on the ignition. The problem with this model is that the power was starting to overcome the handling.

The logical progression was the true GPz550 of 1982 with Uni-Trak rear end. Wheelbase and rear wheel travel both went up, and the forks got air assistance and dual rate springs.

They squeezed another 3bhp out of it with 28mm instead of 25mm inlet ports and Tekei carbs up to 26mm. Finish was better, and you can identify this one by the five-spoke wheels and black tail-light, it had the sidestand cut-out switch, and camchain problems were solved by fitting a Hy-Vo chain with Teflon-coated slippers.

The final model came in 1984. This had the LCD unit set in the petrol tank, the three-spoke wheels and a frame-mounted fairing. Suspension was made easier to adjust and anti-dive went on the forks. By now they had got the hang of black chrome as well.

Despite their eventual fairly high state of tune the CPzs don't seem either fragile or thirsty.

Pre-Hy-Vo chained models could lunch their camchains in only 10,000 miles, but the later versions would go on for much longer.

You can forget the ignition except for an occasional lubing of the mechanical advance.

The later models managed to get more performance and more economy by dint of the lean-bum engines, but these run very hot even with the oil-coolers, so make sure you change the oil often and keep it topped up since they hate running low.

They normally start to smoke around the 35,000 mile mark, but can normally go on to 50,000 before you need a top-end overhaul, which is normally for worn valve guides and piston rings.

A major area to inspect is the Uni-Trak rising-rate linkages, if you are going for a later model. These have a highly stressed linkage arrangement and it needs frequent lubrication, stuck, as it is, down where grit and salt can attack it To check it stand at the back of the bike and lift it up and down. If it is worn you will hear a clunking when you change direction. The handling will be very rough and choppy. Note that there need not be any lateral movement as the swing arm pivot may be unaffected.

Our GPz, a California-only model, arrived equipped with its mandated tangle of paraphernalia for trapping evaporative emissions: a collector bottle under the right side panel to gather condensed fuel droplets from the tank, plus a charcoal canister mounted in the tail section, all connected by yards of rubber hose coiling in and out around the chassis. Kawasaki representatives assure us that performance of the California model is on a par with that of the so-called 49-state bikes; however, West Coast GPz owners are advised that accidentally pinching the fuel fume hoses may damage the ventless gas tank when a full load of fuel presses against the tank under heat expansion.

The greatest boon to the rider with touring on his mind is the GPz's incorporation of the ZX series' rubber-mounting system, which isolates the engine in a set of four elastic dampers to quell vibration. Last year's bike with a solid-mounted engine proved an-noyingly buzzy when ridden above 6000 rpm for anything longer than a sprint. Aboard the new 550, an irritating hum still buzzes through the bars

at precisely 60 miles an hour in sixth gear, but this must be attributed to a maverick harmonic; at virtually every other speed and rpm, the rubber-cradled GPz is remarkably smooth and vibration-free.

While styling and engine alterations are the most obvious changes to the '84 GPz, much of the new model's face-lift is centered in its chassis. Here changes abound, though it takes a keen eye to ferret them out. In an effort to keep pace with the new breed of sporting middleweights with 16-inch front wheels, Kawasaki has quickened up the GPz's steering geometry. The switch from a leading-axle fork to center-axle legs steepens rake by 1.5 degrees and shortens trail by more than half an inch. Stiffer spring rates give a tauter ride, and new-style 18-inch wheels fitted fore and aft replace last year's 19-inch front and 18-inch rear spinners. Though the new GPz still loses ground through tight transition turns to Suzuki's hyper-fast GS550 and the new Honda 500 Interceptor, its newfound agility, excellent ground clearance and high-spee ahead of last year's GPz in overall sporting prowess.

The new fork also carries anti-dive units adjusted by cranking a knurled knob at the bottom of the fork leg. Activated by increased hydraulic pressure when the brakes are applied, the hardware added little to fast-riding control. Unlike many other hydraulic systems, however, the units do not compromise braking action by creating a spongy sensation at the lever. Stopping action is linear and progressive front and rear on the GPz, though the Kawasaki's dual front discs cannot match the power and controllability of the dual-piston calipers mounted on the Suzuki or the Honda 500 Interceptor.

Like the other ZX models in the GPz line, the 550's Uni-Trak rear suspension system's gas-charged Kayaba damper is now carried in forged aluminum arms, and the shock bolts directly to the frame at its upper end, allowing the rocker arm and linkage to be positioned beneath the extruded, box-section aluminum swing arm for a lower center of gravity. Wheel travel is somewhat reduced by this system, spring rates are up, but the suppleness and load-handling range of the GPz's new rear end give it compliance and stability to match that of the latest ad weaponry. Damping rates are se-jcted by turning a wheel at the base of the shock body, and preload changes are eased by the use of a remote hydraulic adjuster mounted just under the left side panel.

If you're getting the impression the new GPz is in every fundamental way " better motorcycle than its predecessors, you're right. Deft and careful ands have corrected this bike's ortcomings while preserving the strengths that have fostered a cult of GPz fanatics, and the precepts of function have been followed at every turn.

For years, riding a GPz was as close as you could get to owning a piece of the sport-bike rock. The 550 ruled as uncontested king of the middleweight jungle, winning its spurs in canyon battle, often against larger foe. Now the GPz's toughest challenge comes from its rivals in the middleweight camp; the GS550 is quicker, the FJ600 stronger, the 500 Interceptor more advanced.

Designs have inherent limitations, and refinement risks annihilation in a period of exploding change. Ultimately, the only law the sport-bike community observes is survival of the fittest, and the fittest is the fastest in a world the GPz550 did much to create.