Yoshimura: The Motorcycle Tuner Who Created Monsters from Machines

You don’t duck down behind some sort of wimpy, wind-cheating fairing on the Pops Yoshimura-tuned Suzuki GS1000. You sit up, reach out for those high, wide bars, and hold on tight as this stripped-down bike with its inline-four engine tries to peel you off the seat at 150 mph.

Superbikes like the GS1000 ruled throughout the 1970s and into the ’80s, but now they are seen as crude lash-ups of dead-end engineering and worse fabrication. The truth is, these Japanese-built naked bikes were monsters. The great age of Superbikes began with the ’69 Honda CB750K and its inline four-cylinder engine, and some 61,000 examples of the quad-exhaust Honda were already on the road in America by 1971. This prompted Hideo “Pops” Yoshimura to leave his small motorcycle racing shop in Tokyo in 1971 and move to America, establishing www.yoshimura-rd.com/to build aftermarket exhaust systems in the San Fernando Valley, a vast suburb adjacent to Los Angeles where everyone rode a motorcycle when they weren’t working in a factory and assembling a Chevy Camaro, Lockheed L-1011 airliner, or ICBM rocket engine.

Yoshimura quickly made a name for himself with the first-ever, four-into-one exhaust pipe, which not only improved the CB750K’s power but also reduced weight. For all that, it took the arrival of the Kawasaki Z1 in 1973 to get the Yoshimura Superbike era really rolling. Lewd and crude, the Z1 made “speed wobble” a common phrase. Yet its 81-horsepower, 903cc inline-four impressed. In the spring of 1973, a fully-faired Yoshimura Z1 with its engine tuned to deliver 105 horsepower at 10,000 rpm set a closed-course speed record of 160.19 mph on the Daytona International Speedway’s tri-oval.

The Z1 transformed amateur road racing in America and led the American Motorcyclist Association (AMA) in 1976 to establish a formal racing class for “Superbikes,” modified production bikes with an engine displacement of 1000cc. Pops Yoshimura struggled to get his Z1s to the finish at the big race at Daytona in 1976 and 1977. Then, in 1978, Suzuki introduced the GS750. In just 50 days, Pops built a 1000cc superbike version of the Suzuki, and Yoshimura R&D America team rider Steve McLaughlin rode it to a dramatic victory at Daytona.

The next year Suzuki introduced America to the GS1000 and its air-cooled, 997cc inline-four engine, and it became the superbike template for Pops. The DOHC, eight-valve cylinder head had stout, bucket-type valve actuators and clean-burning hemispherical combustion chambers. With a built-up roller-bearing crankshaft, the engine made 88 horsepower at 8,200 rpm.

Then Pops turned it into a 150-mph screamer. His GS1000 engine incorporated lightweight connecting rods and special low-friction pistons engineered for extra strength and cast from a heat-resistant aluminum alloy. The cylinder head was ported and polished, then in went hot camshafts, unique undersize buckets, specially made steel valve springs, and oversized valves. As ever, Pops specified his favorite 31mm Keihin carburetors, and the result was a reported 133 horsepower at 10,000 rpm.

...the wide, heavy engine had a high center of gravity and limited cornering clearance, so Cooley had to adopt an acrobatic riding style, especially in slow corners. He would dive the GS into corners, braking late and relatively upright into the apex, square off the corner at low speed, and then whack open the throttle to power down the straightaway.

In those days before computer-aided design and five-axis mills, Pops simply hot-rodded every aspect of the engine by hand, testing his improvements on a Japanese-built engine dynamometer. “Pops would come into the shop and spend the whole day doing dynamometer runs while a couple of helpers fabricated new pieces for him with simple tools,” recalls Joe Weinroth, a member of Yoshimura R&D America during that time. “The only time he’d come into the office was at lunch, when his wife would pedal her three-wheel bike about a mile from their house and bring him a home-cooked Japanese meal. He didn’t really care about selling street pipes; he just wanted to win races.”

Then Pops turned it into a 150-mph screamer. His GS1000 engine incorporated lightweight connecting rods and special low-friction pistons engineered for extra strength and cast from a heat-resistant aluminum alloy. The cylinder head was ported and polished, then in went hot camshafts, unique undersize buckets, specially made steel valve springs, and oversized valves. As ever, Pops specified his favorite 31mm Keihin carburetors, and the result was a reported 133 horsepower at 10,000 rpm.

In those days before computer-aided design and five-axis mills, Pops simply hot-rodded every aspect of the engine by hand, testing his improvements on a Japanese-built engine dynamometer. “Pops would come into the shop and spend the whole day doing dynamometer runs while a couple of helpers fabricated new pieces for him with simple tools,” recalls Joe Weinroth, a member of Yoshimura R&D America during that time. “The only time he’d come into the office was at lunch, when his wife would pedal her three-wheel bike about a mile from their house and bring him a home-cooked Japanese meal. He didn’t really care about selling street pipes; he just wanted to win races.”

Pops Yoshimura’s secret weapon proved to be Wester Steven Cooley. After Steve McLaughlin left the Yoshimura team, Cooley took over as the lead rider just as the GS1000 arrived. In the 1979 Daytona Superbike race, Yoshimura R&D America’s three GS1000s swept the podium, and Cooley’s unfaired bike went through the speed traps at 167 mph on his way to a second place finish. Cooley prevailed in the 1979 AMA Superbike championship, although he didn’t win a race. At the 1980 Daytona Superbike event, one of the Yoshimura team’s Suzuki GS1000s —now equipped with a bikini fairing homologated by the famous, limited-production, blue-and-white GS1000S — won, and Cooley again went on to win the Superbike season championship. At the 1981 Daytona Superbike race, Cooley once again found his way to the top step of the podium.

The Suzuki GS1000 was the best-handling big street bike of its time thanks to frame geometry that enhanced stability with a long 59.3-inch wheelbase and relatively slow steering. Even so, this was a 502-lb bike (dry) on relatively narrow tires, and it needed some improvements to race at 150 mph. Pops and Suzuki collaborated to improve the chassis with upgraded suspension damping, lots of frame gussets for added structural rigidity, and a re-engineered swingarm. Even so, the wide, heavy engine had a high center of gravity and limited cornering clearance, so Cooley had to adopt an acrobatic riding style, especially in slow corners. He would dive the GS into corners, braking late and relatively upright into the apex, square off the corner at low speed, and then whack open the throttle to power down the straightaway. In 1986, Cooley told Cycle Guide’s Dain Gingerelli, “I like coming off the turns and grabbing a whole handful and having the front wheel come off the ground, sliding it a little bit and then running that thing around like a unicycle.”

With the success of Pops Yoshimura’s Superbikes, motorcycle makers began to support the sales of street bikes with racing programs. Their mantra became, “Win on Sunday, Sell on Monday.” Kawasaki hired dirt-track tuner Ron Muzzy, who put his 150-horsepower Z1 engine in a bike for a couple of unknown dirt-track riders, Eddie Lawson and Wayne Rainey. Honda built its factory-engineered RC1000, then hired its own dirt-track rider, Freddie Spencer. The result was a golden age of Superbike racing that ultimately led Lawson, Rainey, and Spencer to the Grand Prix racing circuit in Europe on 500cc two-strokes, where they dominated with multiple world championships.

When economic times got tough in 1981, Pops went home to Japan and his son Fujio took over the U.S. business. Pops passed away in 1995 at the age of 73 and was inducted into the AMA Motorcycle Hall of Fame in 2000. Wes Cooley suffered injuries in a 1985 crash that forced him to retire from motorcycle racing, and he too was later inducted into the AMA Motorcycle Hall of Fame. “Cooley wasn’t just fast as hell on a road racing bike; he was relentless and unafraid, too,” recalls Mitch Boehm, a long-time editor at Motorcyclist. “That was a pretty brutal combination for anyone lining up against him. I raced against him at a few 24-hour endurance events toward the end of his career, and I remember the way he once slithered by me in Turn 8 at Willow Springs International Raceway at 3 a.m., doing about a buck-thirty in the dark.”

Cooley wasn't just fast as hell on a road racing bike; he was relentless and unafraid, too. That was a pretty brutal combination for anyone lining up against him.

And yet the great age of stripped-down Superbikes has not been forgotten. Last year, AMA Vintage Motorcycle Days celebrated the 40th anniversary of the Superbike racing category and invited Wes Cooley to attend. Cooley’s classic, red-tank Yoshimura Racing Suzuki GS1000 was there as well, thanks to collector Brian O’Shea. “When riders in Australia or Europe build a bike for vintage racing, they all want a Yoshimura-style Suzuki,” O’Shea says. “Even the two books published long ago to celebrate Yoshimura’s racing history are worth more than $200 each on the collectible market.”

All this seems like a long time ago, because today, street-legal, track-ready motorcycles are commonplace. But when we look at the Suzuki GS1000 built by Yoshimura R&D America, it reminds us of a time when naked Superbikes really were monsters. It also brings us back to a time when fast, tough-guy motorcycles were created not by a squad of factory engineers, but by a dedicated, visionary tuner — one man, one toolbox, and one very memorable motorcycle.