In 2017, a fascinating story surfaced about one of the most iconic moments in flat-track racing history—the 1975 AMA Indy Mile, where Kenny Roberts famously rode the Yamaha that changed the game. Yet, controversy still lingers over the true specifications of the bike Roberts piloted to victory, particularly the engine that powered it.

For decades, the bike was widely reported to house a Yamaha TZ750 engine. Every article, feature, and retrospective has upheld this narrative. However, a critical detail unearthed by the late Stephen Wright, a well-known motorcycle collector and the owner of the historic bike, suggests otherwise.

During his restoration of the bike shortly after purchasing it, Wright made a groundbreaking discovery: the engine wasn’t a TZ750 but a TZ700. Specifically, its top end was built from two TZ350 engines. This revelation has been verified by Wright himself before his passing in 2014.

It’s hard not to wonder if this detail would have mattered to Harley-Davidson, who had already been humbled by Yamaha at Indy in ’75. Perhaps knowing the engine was 50cc smaller than advertised would have added salt to the wound.

Another point of contention lies in the bike’s horsepower. Most reports over the years have cited the engine at 125 rear-wheel horsepower (RWHP). However, insiders like Yamaha’s Dennis Mahan argue that the figure was closer to 140+ RWHP. Even Roberts himself mentioned in a 2009 Indy Mile video that the bike had “almost double the horsepower” of the alloy XR750s it raced against.

To put this in perspective, a Grand National-caliber XR750 in 1975 was producing around 75 RWHP. Engines built by legendary tuner Jerry Branch often surpassed Harley-Davidson’s official Race Department engines. Branch, the genius behind Branch Flowmetrics, developed port configurations that outperformed H-D’s specs by 3+ horsepower—a testament to his engineering prowess.

Stephen Wright wasn’t just a collector; he was a steward of motorcycle history. His impressive collection included iconic race bikes, some of which were displayed in the AMA Hall of Fame Museum in Ohio. Among them was Mert Lawwill’s 1969 Grand National Championship-winning Harley-Davidson KR.

Wright’s discovery about the Roberts Yamaha serves as a reminder of the complexities and intrigue surrounding motorsport history. It’s not just about the machines or the races—it’s about the stories, the egos, and the pursuit of perfection that define this exhilarating world.

As the debate over the TZ750/TZ700 continues, one thing is certain: the 1975 Indy Mile will forever remain a pivotal moment in flat-track racing, fueled by innovation, rivalry, and the unrelenting spirit of competition.

Watch this video from 2009 when Kenny Roberts rode that bike once again at Indy Mile in front of an enthusiastic crowd: