Yoshimura history – 10

The cover of the 1977-1978 Yoshimura Racing Parts Catalog features Wes Cooley at the 1977 AFM Ontario 6-Hour Endurance Race, which he won together with his teammate Tony Murphy. Unlike the Yoshimura Kawasaki Z1’s for AMA Superbike, the Z1 that Yoshimura built for the race had a 4-into-1 exhaust, since the race was an open class event. Cooley earned his first AMA Superbike win in the season’s final round at Riverside.

Yoshimura History #10: Taming the Four-cylinder Monsters

1973-1977: The Early Days of AMA Superbike Racing

While Yoshimura’s U.S. subsidiary was taken over by its stateside business partners, the motorcycle race scene has reached a new historic milestone on July 28, 1973, as the first AMA production road races took place alongside the Laguna Seca (California) round of Road Race National.

The first AMA production road races, consist of Open Production Class and Lightweight (350cc) Production Class, were held on the Saturday of the race week, a day before the final of the main race. The production race regulations required all bikes to retain the stock appearance, including the configurations of exhaust pipes, brakes (dual front disc conversion was a no-go) and instruments. Engine, however, may be modified internally but carburetors must be stock.

The top two spots in the Open Production race were swept by Kawasaki Z1 riders, with Yvon Duhamel of U.S. Kawasaki factory team taking 1st and Steve McLaughlin taking 2nd. McLaughlin, who later played a key role in establishing AMA Superbike Championship in 1976 and FIM Superbike World Championship in 1988, was also the main person behind this race. The 3rd place bike was the BMW R75/5 ridden by Reg Pridmore. The race turned out to be a great showcase of new 750-class sport bikes as it was putting Z1, BMW, Honda CB750 Four, Kawasaki H2 (two-stroke triple), Triumph, Norton and Laverda together on a single racetrack. Lightweight Production race was mainly composed of Yamaha RD350’s.

A list of performance engine parts for Kawasaki Z1/KZ1000, complete with installation time estimates, found in 1977-1978 Yoshimura Racing Parts Catalog. The Road Special Cam and the Bonneville Special Cam are respectively the equivalents of today’s ST-1 Cam and ST-2 Cam.

The power curve is that of 998cc 69.4mm piston kit (for AMA Superbike) which produced more than 130ps when combined with 31mm Keihin CR’s and a 4-into-1 pipe. The 987cc kit uses 69mm pistons.

The catalog also listed 31mm Keihin CR carb kit, 29mm Mikuni smooth bore carb kit and a supremely practical close ratio transmission kit.

There were two types of 4-into-1 exhaust pipes: machine-bents for street use and hand-bents for track use. Although the catalog only contained 6 pages, it was like a bible to the Z1/KZ1000 riders.

During the early 1970’s in the United States, production motorcycle road racing was quickly gaining popularity among performance bike owners –––– especially among those who bought Japanese 4-cylinder bikes such as Honda CB750 and Kawasaki Z1 –––– as an opportunity for them to try their bikes out on a track. Technical rules for production racing varied from event to event, but it was obviously also an opportunity for Yoshimura to show off their forte.

Reflecting the trend and the atmosphere of the U.S. market, AMA continued to hold production races alongside the Road Race Nationals. The next AMA Open Production race was held at Laguna Seca in 1974 (won by Yvon Duhamel on a Z1), and in 1975, the Open Production race was held twice, first at Daytona (won by David Aldana on Yoshimura Racing Z1) and then at Laguna Seca (won by David Aldana on Dale-Starr Engineering Z1).

AMA made the Open Production an official Championship class in 1976 to be run at all Rounds of Road Race Nationals, and the newly named Superbike Production Class became the root of today’s Superbike racing such as FIM Superbike World Championship and MFJ Japan Superbike 1000. The AMA Superbike Production Class must retain stock chassis and original silhouette. The engine could be modified as long as it retained the stock stroke –––– capacity limit was increased to 1000cc –––– but it must run stock carbs and stock exhausts. In the case of Kawasaki Z1’s, they could have dual front disc brakes, but must maintain the factory stock mufflers.

The very first opening round of the AMA Superbike Championship series at Daytona International Speedway was monopolized by three BMW R90S’s. They were all tuned by American BMW importer Butler & Smith and, based on an interpretation of race rules, two of them featured Yamaha-esque cantilever monoshock rear suspension. The notion was that the swing arm is not a part of the chassis. It was a clever exploitation of a loophole in the regulation. Steve McLaughlin (R90S with twin rear shocks) won the race in a photo-finish over Reg Pridmore (R90S with a rear monoshock).

Another sensation of the race was Wes Cooley riding a Yoshimura Kawasaki Z1, who started from the last row of the grid but managed to finish 4th. The bike was tuned by Pop himself at the Yoshimura R&D of America, which he established in North Hollywood on June 1, 1975. Cooley, who at that time a student in medical field, and whose father was a racer and a racing club (AFM: the American Federation of Motorcyclists) organizer, just had become the first contract rider for Yoshimura.

Always cheerful and friendly young Southern Californian gentleman, Cooley was a natural Superbike rider. Once he gets on a racetrack, he aggressively hangs off his bike and drags his knee at every turn, then with controlled tail slides, he opens up the throttle quicker and wider than anyone else. He never flinches, even when his bike tries to shake him off at the notorious 18-degree and 31-degree high-speed banks of Daytona. Meanwhile, the Yoshimura Kawasaki Z1 was such an unruly beast of a machine. The bike was virtually invincible on straights with a more than 20kph higher top-speed than any other bike on the track, but in corners, its chassis flexed and weaved tremendously, so it could easily be passed by the twins –––– namely, BMW boxers and Ducati 750SS’s –––– with much better braking and handling. Yet, Cooley just kept pressing hard all the way to the finish, to an extent which made someone say, “Cooley has the eyes of a gunslinger.” As the rounds went by, race fans began to call Superbike racing a “wobble parade” or a “courage test for daredevils,” referring to the brutal behavior of 4-cylinder Superbike machines.

Part of the excitement of the early Superbike racing was the battle between 4-cylinder bikes and 2-cylinder bikes, in which the stock Z1 chassis was just too soft for the tuned engines, while the twin supersports of less than 100ps had great handling characteristics. What made it more entertaining was that both bikes and riders had a kind of wildness in their looks, and all the large-capacity 4-stroke engines made such a heavenly music!

The 1976 AMA Superbike Championship season ended with 3 wins for BMW, 1 win for Moto Guzzi and 0 win for any of the 4-cylinder bikes. Although Cooley missed the Superbike podium that year, finishing 4th in Rd.1 at Daytona and 5th in the Rd.4 at Riverside, he proved his talent by winning an AFM road race at Riverside in October.

Pop, who had been tuning not only engines but also motorcycle chassis since his days in Gannosu, was well aware that the big Kawasaki has issues with the chassis. Nevertheless, the AMA Superbike regulations and the suspension technology of that time did not allow him to improve the chassis so as to match the power the Yoshimura-tuned engine was putting out. The momentary absence of Fujio, who had to return to Japan due to visa issues, was making the situation more difficult.

Fujio came back to the U.S. with a valid working visa in November of 1976, and Yoshimura R&D was now ready to enter another Superbike season with Kawasaki Z1’s, at least until the new Suzuki 750s (which Pop had been introduced to a little earlier) arrive. Then, a disaster occurred. On February 18, 1977, while they were preparing for the season-opener, Pop received severe burn injuries from a ferocious fire that took down his workshop in North Hollywood, in which he desperately prevented an explosion by carrying a fuel tank out of the flaming building in his arms. It took a month of hospitalization with painful procedures of skin-grafting and regeneration –––– not to mention a lot of his wife’s selfless devotion –––– to cure the burn which extended from his arms to his face.

1977 AMA Superbike Rd.1, Daytona. With the precious help of supporters and staff, Fujio managed to prepare for the event in less than a month after the fire. The smile on the face of Fujio (at right) explains it all. The man on Wes Cooley’s #33 Z1 is mechanic Suehiro Watanabe who later becomes the vice president of Yoshimura R&D. Don Emde’s #25 Z1 was tuned by Mac Kambayashi. Note that both retain separate mufflers.

Fujio, too, took a hard hit. After losing workshop and Z1’s to fire, he once decided to withdraw from the season-opening Daytona round. But to his surprise, every Superbike rider who visited or called them in sympathy –––– not just Cooley but also rivals such as Pridmore and Neilson –––– asked them to race the season-opener, insisting that Yoshimura makes the Superbike event more fun for everyone. Local tuners and machine shops also came forward to support Yoshimura with building race bikes.

“They were rivals and comrades at the same time. I was so grateful for their kindness,” Fujio recalls.

As clearly stated in the rulebook, the AMA Superbike Championship was established to attract the interests of both competitors, aftermarket suppliers (such as Yoshimura) and fans as well. Its foundational principle implies that the central figures of the event are not those who manufacture motorcycles, but those who ride them, who tune them and who watch them compete. Every round is made possible through the collaboration between them, who therefore share the sense of both rivalry and comradery with each other. The fraternal atmosphere surrounding the Superbike scene is strong, especially during its early years. And the location, too, was on his side.

“There you can find whatever you need all within an hour’s drive,” explains Fujio.

Suppliers of mechanical hardware ranging from fasteners to aircraft parts, engineering services ranging from machining to welding, pro hot rodders, aircraft builders, marine engineers –––– they were all right there in Southern California where cutting-edge industries gathered. By gratefully accepting both the support of people and the convenience of SoCal area, Fujio managed to enter the season-opener on March 11.

For the 1977 season, the AMA Superbike calendar expanded to 7 rounds in total. The Rd.1 win went to Cycle magazine editor/racer Cook Neilson aboard the 883cc Old Blue Ducati 750SS –––– aka the California Hot Rod –––– which he, together with his colleague Phil Schilling, tuned to attain a top speed equivalent to Kenny Roberts’ TZ250. Wes Cooley aboard a Yoshimura Kawasaki Z1 led the race in the early laps as he clocked the fastest speed of the round at 246.27kph, until he began to suffer from suspension failure which slowed him down to finish in the 3rd place.

Rd.2 took place at Charlotte Motor Speedway in North Carolina. The round winner was Mike Baldwin aboard a Moto Guzzi 850 Le Mans. Cooley again finished 3rd after suffering the same problem. In Rd.3 at Bryar Motorsports Park (now New Hampshire Motor Speedway), the victory went to Ron Pierce aboard a BMW R90S. Rd.4, at Sears Point Raceway (now Sonoma Raceway) in California, was won by Paul Ritter aboard a 860cc Ducati 750 Sport, and a Yoshimura Kawasaki Z1 ridden by Steve McLaughlin retired from the race due to a disconnected plug wire. In Rd.5 at Pocono International Raceway (now Pocono Raceway) in Pennsylvania, Reg Pridmore aboard a Racecrafters Kawasaki KZ1000 brought the first AMA Superbike victory for 4-cylinder bikes. A Yoshimura Kawasaki Z1 ridden by Cooley retired from the round due to a valve failure.

Wes Cooley’s Yoshimura Kawasaki Z1 with the set-up dialed in for 1978 AMA season. Until this season, Yoshimura machine equipped with a 4-into-1 pipe could only be seen at AFM road races at Ontario and Riverside. 29mm smooth bore Mikunis had to stay there for another season until they were replaced by Keihin CRs, but the bike sure looks better with a 4-into-1! Note that some race bikes, including this Z1, had to run a treaded tire on the front and a full-slick on the rear to prevent wobbles.

While facing various issues in the AMA Superbike races, Wes Cooley / Tony Murphy brought a victory to Yoshimura Kawasaki Z1 on April 24 at 1977 AFM Ontario 6-Hour Endurance Race, the biggest club race event in the West Coast at that time. By having the modification freedom of AFM open class, Yoshimura did a full tune on the Z1 with a big-bore kit, CR carbs and a 4-into-1 exhaust pipe, all of which were strictly forbidden under the AMA rules. In the meantime, Suzuki’s presence quickly became salient in this race through the achievements by privateers’ 845cc GS750 (7th overall) and GS550 (550cc Class winner). Yoshimura Suzuki GS750 Superbike was at the time under rapid development, but made its debut win on September 11 in Rd.6 of 1977 AMA Superbike at Laguna Seca Raceway (now WeatherTech Raceway Laguna Seca) with Steve McLaughlin at the controls. And finally, in the season’s final round on October 1 at Riverside International Raceway, Wes Cooley earned his first AMA Superbike victory with a Yoshimura Kawasaki Z1. The year 1977 was such a dramatic year for Yoshimura, but let’s go back in time in the next chapter to understand what really happened back in 1976.

Yoshimura Z1 performance tuning kits were also popular among drag racers. Piston kits for Kawasaki applications were available in: 69mm (street), 69.4mm (AMA Superbike) 70mm (1015cc, stock KZ1000 pistons) and 73mm (1105cc, drag racing and AFM road races).


Published on March 23, 2020

Stories and photos supplied by Yoshimura Japan / Road Rider Archives

Written by Tomoya Ishibashi

Edited by Bike Bros Magazines

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