1984 Yamaha RZ500: Short-lived sensation

The Yamaha RZ500 was an ephemeral model, lasting only a couple of years in production as the new-generation of four strokes came on stream and emission laws, particularly in the USA, became more stringent.

“Hanging on the wall in my garage, I had this photo of Richard Scott on the RZ500 in the 1984 Castrol Six Hour Race. I looked at it every day for years and then I thought, I’ll have to find one of those.” – Peter Addison

The 1984 race itself had been dramatically uprooted from its home of 14 years, Amaroo Park, to Oran Park, with the announcement coming less than one month before the race. Months of testing at the tight Amaroo track had shown the RZ500 to be a strong contender for outright honours, but the move to Oran Park, with its long main straight, shifted the odds in favour of the VFR1000 Honda and the GPz900 Kawasaki.

The race was a highly entertaining battle between the Dowson/Scott RZ500 and the VF1000R Honda of Wayne Gardner/John Pace. The ding-dong battle looked set for a nail-biting finish at 4 pm, with both bikes running perilously low on fuel as the final seconds ticked by. Then came high drama. With the RZ just ahead on the road, the chequered flag came out 2 minutes and 50 seconds (or about two laps) early! Scott swept across the line to victory, but barely made it back to the pits on the slowing down lap, with the tank dry. The Mentor Honda team was, quite naturally, furious, but their machine also had only a cupful of gas left, so had the race run its full course, the result could have gone either way.

The RZ500 itself had been announced to the world earlier in the year – a machine as close to a genuine 500 cc GP racer as it was possible to put on the road. First tantalising glimpses of the new V4 came at the October 1983 Paris Salon (Motorcycle Show), where journalists described the ‘Kenny Roberts Replica’ in rapturous terms.

Initial Australian deliveries of the RZ500 began in April 1984, and it quickly went into service in the booming Production Racing scene, taking a 1-2 first time out at the Hub 300 at Lakeside in Queensland with Michael Dowson aboard the winning machine and Richard Scott 32 seconds behind him. It was the first victory for the RZ500 anywhere in the world, and it would not be the last. One factor in the win was the RZ’s frugal fuel consumption – it recorded just 21.6 mpg or 7.64 km/l in the race.

Peter Addison (left) and the wall poster of Richard Scott (right) aboard the RZ500 in the 1984 Castrol Six Hour Race.

A racer for the road

Three different versions were listed – a square section steel framed version for Australasia and Canada (the RZ was never legally sold in the USA due to anti-pollution laws), the European model (sold as the RD500LC) which was mechanically identical but with a different décor, and the de-tuned, aluminium-framed job (called the RZV500R) for the Japanese market. At the heart of the matter was a twin-crank 50-degree V4 with Yamaha’s YPVS exhaust power valves, which was strictly speaking not a V4 at all, but a pair of 180-degree parallel twins splayed 90-degrees and gear-coupled together. The close-ratio, six-speed cassette-style gearbox was designed for quick removal to change ratios. The design owed a great deal to the works OW70 raced by Kenny Roberts in the 1983 GP season, but had many differences to make it suitable for the road. Instead of the racer’s rotary disc valves, the RZ500 used reed valves (standard RD400 items) for a broader powerband. Uniquely, the front cylinders were fed through the crankcases, while the rear cylinders received their mixture directly into the barrels. On the OW70, the carburettors were located in a bank between the V, but the RZ had its carburettors cranked out to the side, connected to the reeds by 90-degree manifolds. Barrels and heads were cast in pairs, not individually as on the racer.

Between the two crankshafts, a counter-rotating balance shaft was fitted in an effort to smooth out engine vibration, and this shaft also drove the coolant pump. Similar to the system used on the RZ350, the electric servo-controlled YPVS power valve improved low and mid-range power by progressively raising the exhaust ports. As on all big-capacity two strokes, the expansion chamber exhausts required some complex routing. The lower two pipes sat underslung in conventional fashion, but the top pipes, exiting from the rear cylinder exhaust ports, were wrapped around each other so that the required volume could be achieved without having the pipes sticking out the back of the bike, and a very neat job it was too.

En route to a narrow victory at the 1984 Castrol Six Hour Race, Richard Scott hustles the RZ500 onto the Oran Park main straight.

To a large extent, the tangle of exhaust pipes (and the VPVS servo motor and battery under the seat) dictated that the rear single shock, which in normal practice would have been located vertically behind the engine with a bottom linkage, had to be deployed horizontally under the motor. The forward end of the shock connected to the frame and the engine via a rigid cross bracket, while the rear end, with its spring-preload adjuster, was attached to the aluminium swinging arm through a forged aluminium rocker. Up front was a set of fairly conventional non-adjustable forks with 37 mm tubes, with an external anti-dive mechanism (identical to that used on the FJ1100) on the sliders. At the time, the racing world was going all gooey about 16-inch front wheels, and the RZ500 had one, as much for its marketability as for any performance issue. At the rear was an 18-incher with a 130/80 tyre. The 26-degree steering head angle, plus the small diameter wheel, made the steering fairly vague at low speeds, but quite sharp once you were on the move. The other factor affecting the steering and the overall handling was the RZ’s weight – 180 kg dry (the alloy-framed Japanese market version was a handy 9 kg lighter). A major contributor to the porkiness was the exhaust system, particularly the complex rear cylinder pipes, which meant the weight was carried high in the frame as well. The top pipes were double-skinned in an effort to reduce heat finding its way to the rider’s lower regions.

Despite all the hardware underneath, the seat height is quite low, although very wide, as is the rear of the fuel tank. Once the ignition is switched on, the powervalves automatically perform a rotation to un-gunk themselves. A single-bolt fixing at the front of the fuel tank allows the unit to pivot upwards, giving easy access to the sparkplugs, which on a two-stroke, even a modern one, is a handy measure.

Modern mods

Peter Addison bought his RZ500 in 2008. “I found a very original bike in South Australia. It had been in a shed for about 16 years, but with a new battery and a clean out of the fuel system, it started up and ran quite well. It had only done about 30,000 kilometres.” Peter says he isn’t a diehard purist when it comes to restorations, and he’s done a few. His RZ bristles with modifications that greatly enhance the bike’s performance without compromising its integrity.

“I wanted the bike to look as much like original as possible, because a lot of people turn these into YZR replicas where they really don’t resemble the original at all,” he says. “One area that can really make the RZ much nicer to ride is to change the suspension, wheels and brakes. It’s very hard to get decent tyres in the original 18-inch rear and 16-inch front sizes, so the usual mod is to fit 17-inch wheels front and back, which is what we did.”

Left: Yamaha R6 front brake and calipers. Right: Rear stopper is from a Yamaha R6.

Breathing is another area – air in and air out. The standard RZ intake system is a complicated system of plastic mouldings, beginning with the air filter itself which is located in a box between the top frame rails, then working its way to the side-mounted carburettors through a series of chambers. On Peter’s RZ, spanner-man Dudley Lister, who has a motorcycle business in Wyong on the NSW Central Coast and is a veteran of numerous race teams of the ‘70s and ‘80s, has dispensed with this system and replaced it with a pair of K&N filters of Harley-Davidson specification, which fit (just) inside the fairing and don’t look out of place at all. From the moment the RZ/RD first appeared, expansion chamber makers enjoyed a boom market, constructing lighter and more efficient methods of dispelling the burnt gases. On Peter’s bike, the pipes are made by the English firm Jim Lomas, with both lower pipes exiting on the right side, as on the GP bikes. The top pipes no longer cross over themselves, but still terminate through the neat standard cowling that also houses the tail light.

The Yamaha RZ500 looks good from any angle.

A major change is at the front end, with a set of forks that Dudley had laying around, which were originally used on one of the local Yamaha team YZF1000 Superbikes in the late ‘80s. These forks and the triple clamps are machined from billet aluminium, and not only work significantly better, but further contribute to the weight saving. A nice thought that was developed for the local Superbikes was the quick-release front axle. Brembo four-piston callipers sourced from a Ducati Monster now grace the front and rear ends, with mounting brackets specially made by Lister, who also made a modified a set of Kelvin Franks clip-on handlebars to fit the larger diameter fork tubes. A sensible move to take advantage of modern rubber is the switch to 17-inch wheels at both ends, which came from a Yamaha R6, as did the disc rotors.

The swinging arm was also sourced from Dudley’s ex-race stock, but needed quite a bit of work to make it fit. “The standard swinging arm was not wide enough to accept a modern 17-inch rear tyre,” explains Peter, “but fortunately Dudley had the solution. Originally this swinging arm used a vertical shock, so to fit the RZ’s horizontal shock Dudley had to cut off several brackets and make a new system underneath, with adjustments to lift the ride height at the rear. I bought most of the other bits and pieces on Ebay from the States, and with other little things like smaller blinkers, lighter brackets and so on, it amounted to nearly 30 kilos saved, which makes a huge difference to performance and handling.”

Riding the RZ

Larger, taller chaps found the RZ a bit cramped, with the low seat height and highish footrest positioning, clip-on handlebars and hump-back single seat (a pillion perch was provided by removing a cover, but it was rarely used!). Naturally, steering lock was also rather limited – a chore around town but otherwise not an issue. The RZ relies upon the traditional boot to get it started rather than the push of a button, but one prod was usually sufficient to commence the operation. Symptomatic of two strokes, it needed a decent warm up to clear the innards, after which the V4 idled smoothly and smoked tolerably. Remember that this machine was conceived in the very early days of synthetic two-stroke oils, modern variants of which have dramatically reduced the normal visible smoke trail. The first thing most testers noticed was the very high first gear, but once the revs reached 6,000 everything began to happen rather quickly. Thereafter the RZ fairly rockets up to (and a bit beyond) the 9,500 rpm red line. “It’s peaky, for sure,” says Peter, “but that’s just typical two-stroke and once you get it stirred up it’s fine, in fact it feels quite torquey. And the reduction in weight means it steers and changes direction so much better.” The exhaust note it totally unique – a throaty growl rather than the normal rasp of the smaller two-stroke engines.

“It’s peaky, for sure,” says Peter, “but that’s just typical two-stroke and once you get it stirred up it’s fine, in fact it feels quite torquey. And the reduction in weight means it steers and changes direction so much better.”

Without very close scrutiny, Peter’s RZ looks like it just came out of the factory, but with the mods it is now a much nicer package to ride, and far more practical in many respects. And there’s no doubt that if the halcyon days of the big, bad V4 GP 500s were your thing, then the RZ500 provides all the panache with the added bonus of a package that’s legal to ride on the road. The RZ500 was an ephemeral model, lasting only a couple of years in production as the new-generation of four strokes came on stream and emission laws, particularly in the USA, became more stringent. Yamaha was very keen to push its new five-valve technology in the FZ750, so the RZ was quietly discontinued and consequently is quite rare today.

Peter Addison, occasional road tester for Old Bike Australasia magazine, was Team Manager of the crew that looked after the Richard Scott/Michael Dowson Toshiba Yamaha Dealer Team entry in the 1984 Castrol Six Hour Race, with Steve Ashkanzi as Chief Mechanic.

1984 Yamaha RZ500: Specifications

BORE & STROKE 56.4 x 50.0 mm
POWER 90 hp (64 Kw) at 9,500 rpm
TORQUE 67.5 Nm @ 8,500 rpm
CARBURATION 4 x 26 mm Mikuni.
TRANSMISSION 6-speed cassette.
FUEL TANK 22 litres
FRONT TYRE 120/80 x 16
REAR TYRE 130/80 x 18
TOP SPEED 230 km/h

Story & photos: Jim Scaysbrook • Race photo: John Ford/Motorcycling Australia

OBA Issue 17
This article first appeared in Old Bike Australasia Issue No.17