02. An Engineer Who Never Stops
If the frame is the bike’s skeleton, the engine is its heart—and Kitahara Naotetsu is the coordinator overseeing the full scope of YZ engine evaluation.
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Engine testing isn’t just ride feel. It includes measuring engine-only performance, checking functionality (oil circulation and pressure), evaluating reliability through durability tests, measuring noise to meet race regulations, assessing starting, and more. Kitahara describes himself as a “jack-of-all-trades”—someone looking at everything, managing everything.
His central challenge is blunt: make an engine as fast as possible while keeping the standard quality of not breaking down.
Because the YZ is ridden by a wide spread of riders, Kitahara places particular attention on the low- and mid-speed range—where everyone from professionals to beginners spends time. Making a bike easier to ride can sometimes mean dialing power back, but the goal here is different: create a rider-friendly engine without reducing power more than necessary.
He’s equally demanding about feel. In dirt, “more power” doesn’t automatically equal “more forward motion.” Too much can simply spin and make the bike harder to ride. Too little, and the bike won’t perform. So the team evaluates and develops the balance by riding.
For the YZ250F, which Kitahara mainly works on, that means chasing the drive out of corners—when the throttle is closed through the turn, then opened again and the bike “hooks up” and accelerates. He wants that punch and forward motion to be right there, and he’s determined it won’t be inferior to any other manufacturer.
That pursuit becomes real-world trial and error: changing exhaust pipe shape and length, reworking the silencer, even cutting engine ports by hand. For parts they can’t manufacture internally, the team uses experimental results to guide requests for components with specific characteristics.
Kitahara’s drive to keep trying new approaches is tied to his own background. He’s a former International B-class rider who raced in the All Japan Championship. Earlier in his engineering career, he developed the bike mainly by listening to rider comments. Over time, he felt his mental “framework” becoming fixed—so he changed how he worked to break through.
For roughly the past three years, he’s been riding the YZ himself. He says it has broadened his imagination, helped him understand development riders’ feedback, and allowed him to connect feel with the numbers. It’s physically demanding—and he can’t ride like a professional development rider—but he believes it has brought real benefits. The conclusion he keeps coming back to: the rider’s senses are the most important thing in development.
Because he understands what winning means for the YZ, race results hit hard. Cooper Webb’s 2025 AMA Supercross 450SX title was especially meaningful for Kitahara on a personal level. Webb had raced the All Japan Championship at SUGO in 2014, and Kitahara was helping as part of Star Racing staff at the time. He’d even sent Webb a personal message—so seeing him return to Yamaha, ride a YZ, and win the championship carried extra weight.
Kitahara also reflects on the YZ250F—the first machine he was deeply involved with—and how Webb’s success on it felt like they were “growing together”. With Star Racing continuing to prove the YZ’s strength, and Hayden Deegan carrying that tradition forward, it becomes fuel to keep pushing. And with more young riders coming through, the motivation only grows.
Those ideas flow into the product itself. Webb once told Kitahara that motocross makes life more enjoyable. Kitahara wants the YZ to deliver both training and enjoyment—helping kids and young riders get faster, veteran riders enrich their lives, and professionals win races. He sees the bike as a tool for people to grow in many ways, and believes that’s what drives the entire development process.
Like riders chasing the top step, Kitahara and the YZ developers share a mindset of never stopping—and that, he believes, is why the YZ continues to climb to new heights.