Kawasaki GPz900R
Two faces
The smallest of Japan’s Big Four motorcycle manufacturers has always punched above its weight. For half a decade following its release in 1973, the Z1 and its variants was king of the heavyweights, stunning its rivals into lifting their game to regain market share in the top sales segment. But lift they did, and by the late 1970s the old air-cooled Kawasaki was showing its age, looking a bit portly, and not quite the rocket ship it had once been. Across in Japan, the Big K’s designers were continuing to churn out concepts, but transferring these into metal was another thing. By 1982, Kawasaki’s motorcycle division, which was and is a very small part of the industrial giant’s operations that include fixed wing aircraft and helicopters, trains, industrial robots and ships, was losing money, both in its home operation and the American subsidiary. But far from pulling back, Kawasaki’s answer was to attack, with the primary aim of increasing its share of the home market (the world’s largest) significantly beyond the modest two per cent it held in 1982. To this end, a plethora of new models was planned, from the svelte tandemtwin KR250 (based on the company’s all-conquering Grand Prix bikes), to the new king of the range, the GPz900R (with 550 and 750 cc sub-models).
The GPz900R owed nothing to the Z1 ancestry apart from the fact that it had a wheel at each end. Unveiled to the world’s motorcycle press in December 1983, the GPz900R immediately proved faster than both the existing GPz750 Turbo and the top of the range GPz1100, both of which were based on the old 2-valve Z1 concept. The 900 was completely new, with water cooling for the 4-valves per cylinder DOHC engine, which was employed as a semi-stressed member of the chassis – a first for Kawasaki.
Looking at the engine in detail, Kawasaki’s engineers considered a wide variety of configurations, including a square four, a v-four, a v-six and an allnew in-line six (bearing little relationship to the existing water-cooled Z1300 six) before opting for the relatively ubiquitous in-line four. From the start, the design team worked on the strict direction that the new power plant had to be lighter, narrower and shorter than the old mill. Less mass equals more power, but power in itself is only half the equation. Power-to-weight ratio is where it really counts. The 903 cc Z1 pumped out an astounding (for the time) 82 horsepower, while the GPz achieved 115. Importantly, the new model came in with a power/weight ratio of 0.5 hp/kg – a 60 per cent increase over the Z1. The engine weighed 5 kg less (even including the radiator and oil cooler) and was 123 mm narrower than the air-cooled unit. Due to the efficiencies of the wet-liner cooling (where the coolant runs directly against the pressed-in liners rather than cast-in channels), bore-to-centre distances were able to be significantly reduced from the air-cooled design, reducing the overall width of the block.
In a major departure from conventional (ie; Japanese) motorcycle engine design, the GPz dispensed with the centre-of-crankshaft drive for the overhead camshafts, opting for a chain drive on the left side of the motor. A silent-running single-row chain was tensioned by a new constantload KHI tensioner that Kawasaki claimed to reduce power losses through friction and give increased chain life. An ignition-triggering device was also contained on the left side of the crank. The fivebearing one-piece crankshaft drove the clutch and the counter-balancer via a straight-cut primary gear situated between the third and fourth cylinders, with the gear doubling as an inboard flywheel and also driving an additional shaft, located beneath the crank, carrying balance weights.
The science of cylinder head design had advanced exponentially in the previous decade, with evernarrowing valve angles; the GPz used an included valve angle of 18.5 degrees for the inlet and 16.4 degrees for the exhaust, but with comparatively large valves: 29 mm and 24.7 mm respectively. The hollow camshafts each had four lobes, with each lobe actuating two valves via forked rockers. Gone was the old shim and bucket system to set valve clearance, replaced by screw and locknut adjusters. By removing the centre drive to the camshafts, inlet tracts could be made shorter and more direct, with equal distances between each valve and the corresponding carburettor. These carbs were a new Keihin model, designated CVK34 with a semi-flat slide that allowed the actual carb body to be reduced by 19 mm with a further slight saving in
“In a major departure from conventional (ie; Japanese) motorcycle engine design, the GPz dispensed with the centre-of-crankshaft drive for the overhead camshafts, opting for a chain drive on the left side of the motor.”
weight. Kawasaki admitted that the use of carburettors was mainly a cost consideration, but that fuel injection would be a relatively simple matter for future models.
With the switch to water cooling came extra componentry including thermostat, water pump, electrically-driven fan and the aluminium cross-flow radiator which weighted just 1.2 kg.
Atop the gearbox sat the alternator which further helped the quest for a narrow engine. The gearbox itself was a six-speeder – rare in those days – with a hydraulically-operated clutch.
While the power plant was indeed state of the art, the chassis also embodied some fresh thinking. What Kawasaki called a ‘diamond’ frame was actually a twin-loop top section stretching over the engine, which was suspended below. This gave extra rigidity to the steering head section. At the rear, aluminiumalloy box section sub frames provided outboard support for the steel swinging arm and the rear sub frame. Steering head angle was 29º from the vertical with 114 mm of trail. Inside the front fork was a system to provide progressive damping, whereby compression damping was automatically varied according to both the speed and the front wheel travel. This system Kawasaki designated AVDS (Automatic Variable Damping System), using a secondary spring valve system which automatically metered hydraulic fluid transfer during compression. What this actually meant was the front end was effectively stiffened when under greater load while allowing soft damping under less severe conditions. An anti-dive system operated through a hydraulic link to the brake calipers. The 38 mm forks were also air-assisted with an equalizing set up. At the rear end sat a refined version of the single shock Uni-trak system, also air-assisted, with four adjustments for rebound damping. Adjustments were effected via controls under the right hand side cover.
With such a refined engine and chassis package, the next step was close attention to aerodynamics, and Kawasaki subjected various mock-ups to extensive testing in its wind tunnel before the final shape of the fairing was decided upon. The use of a sixteeninch front wheel (at that stage the favoured set up on GP bikes) lowered the front end, and the slim engine allowed for frontal area to be reduced by around ‚
ten per cent over the GPz750 Turbo. The result was a machine with a very low coefficient of drag, although perhaps in the interests of cooling, the fairing did not follow usual practice of completely enveloping the engine. Despite the spirited performance, the GPz900R was docile in traffic and almost vibrationfree, thanks to the crankshaft counter-balancer. Touring riders praised the efficiency and comfort afforded by the fairing and the plush seat.
The combination of light weight, scintillating performance and excellent handling was not lost on the sporting set, where the GPz900R quickly found favour. The new 900 could top 240 km/h – the first production road bike to do so. At the 1984 Australian Grand Prix at Bathurst, Rob Phillis took out the Unlimited Production Race and the following year Kiwi Richard Scott led home a GPz900R 1-2-3 in the same event. Of course, the race that mattered in Australia was the Castrol Six Hour Production Race, and for 1984 the event shifted from Amaroo Park, its home of 14 years, to Oran Park. The much faster circuit was seen as a gift to the GPz900R teams, of which there were four in the Unlimited class, ranged against four Honda VF1000R teams. For the first hour of the race the Kawasaki filled the first three places – Willing, Paul Feeney and Phillis – but were thereafter beset with problems including rear tyres and overheating, and before the end both Phillis and Iain Pero retired with gearbox woes.
Stung by the defeat, Kawasaki redoubled its efforts for the 1985 Six Hour, and in a race held in sometimes heavy rain, Willing and team mate Pero held the lead for all but the final ten minutes, when a worn out rear tyre sent Willing crashing to earth. Remounting, Willing brought the battered GPz900R home in second place, but the glory of a Six Hour win eluded the model, which by 1986 was outclassed by the new Yamaha FZR750.
Although variations such as the GPz1000RX (1986) and the ZX-10 were introduced, the GPz900R, with minor changes to the front forks, brakes and wheels remained in the Kawasaki line up until 1993 in Europe and Australia, and 1997 in USA.
Today, the GPz900R is slowly gaining classic status, although a major impediment to concours restoration is finding a decent exhaust system. The original mufflers were black-chromed, a process that few platers will undertake. Engines were also known to develop pitting on the camshafts and the later models were fitted with larger diameter external oil feed lines. Despite the water-cooling, overheating was not unknown, and rather than rely on the thermostat, a common modification was to fit a manual switch to activate the electric fan. Like many bikes of the early ‘single shock’ era, the linkages wear, partially due to absence of sufficient lubrication points.
An owner’s opinion
Matthew Bunt, from Mount Gambier in South Australia, is a keen GPz900R owner, describing it as “a fantastic machine.” He found time to run a camera over his bike and pen a few words. “Ownership of My GPZ900 came about after a few years of persistent hassling of a work mate who had it sitting in his garage for 15 years unused. My first visit to his place one evening proved the bike had indeed been sitting for a long time, flat tyres, blown rear monoshock and some dried up pools of unknown fluids that had soaked into the concrete under the bike. Add a lot of dust and it looked like a dog but I could see the gem underneath all the years of neglect. At this stage I must confess I did not have much of an idea of what I was looking at, I just wanted an old sportsbike as a project. After some research I realised the bike was an original 1984 A1 GPZ900R, this made me want it even more. The fact that it appeared in the Top Gun movie and it has gained somewhat of a cult status was lost on me since I was only a kid when the movie came out. “Presuming the bike was going to need some major work to ever get on the road again I made an offer to my mate to which he replied “No. I’m not going to give it away!” At that point in time for me, with a young family and mortgage meant that his price was just a bit out of reach, so I walked away without the bike, certain I would get the extra cash soon. Well life does not work out like you want sometimes, and a couple years and a divorce later I remembered the GPZ and asked the owner again. He started telling me how much of a classic it is and that they are worth heaps and I thought he has decided to put the price up but he said for his original asking price it was mine. This time I had the cash and a deal was done. I loaded the bike on the trailer and got it home.
“The first thing I did was track down a good second hand original rear monoshock which I found in Queensland for $50. Next was a good clean and apart from the usual grime, grease and dust and a slight dent on the tank, what emerged from under the dust was what I had hoped, a pretty good clean bike with an original 32,000km showing on the clock, a bit of a survivor. I booked the bike in to Tim Campbell Motorcycles in Portland Vic, which a friend with a couple of Katanas recommended, with the instructions to get the bike on the road again and do whatever needed doing. To their surprise when they got the bike and went to drain the fuel tank they found the good old Super petrol was still inside. This meant no gummed up carbs and a lot less work to get the bike running. New tyres, brakes, major service and battery and it was up and running and idling smoothly. ‚