First race, first win: Tetsuya Mizoguchi on his way to riding the new KX250F-SR to a perfect win at the opening round of the 2003 All-Japan Motocross Championship.

High-pitched whines blended with deep-throated roars, the cacophony of exhaust notes circling the course. At the crest of this wave of sound was a KX250F-SR adorned with rider plate number 711. Kawasaki's new 4-stroke racer made its debut at the opening round of the 2003 All-Japan Motocross Championship. Tetsuya Mizoguchi (a rider with the Kawasaki Racing Team at the time) was shouldered with the heavy responsibility, but came through with flying colours, winning both motos to give the KX250F-SR a perfect win in its maiden race. Although the IA125 class (now IA2), in transition at the time, fielded many more 2-stroke 125cc machines than 4-stroke 250s, the KX250F-SR's dominant performance in that opening race clearly showed the potential of 4-stroke racers.
Mizoguchi continued to win races and went on to take the 2003 IA125 championship. The following year, Tomonori Nakamura (a rider with Team Green at the time) took the same title, giving the KX250F-SR two championships in a row.
"Race results and positive response from the market both fuelled the 4-stroke movement. The 2004 and 2005 model KX250F was jointly developed with Suzuki. The fact that we were able to capitalise on each other's different company cultures yielded numerous benefits, including development moving ahead more efficiently and much more quickly than we anticipated."

Seiichi Kai: in charge of engine development for Kawasaki's new 4-stroke motocrossers.

Seiichi Kai had years of experience in 2-stroke development, and was in charge of engine development for the new 4-stroke motocrosser.
"The 250 was released first, but in actual fact 450 development started earlier. Considering the strength of various parts, developing the larger model first, then moving on to the smaller model is the correct way to proceed. So development started with the flagship. In 2002 we did a test ride at Hiroshima's Korakuen. We had mounted a 450 test engine in a KX250 frame, because of course maintaining chassis dimensions was one of our prerequisites. A 450cc engine in a KX250 chassis, a 250cc engine in a KX125 chassis… those were the requirements for our 4-stroke development. After numerous tests the KX450F-SR made its debut at the opening round of the 2005 All-Japan Motocross Championship. Nakamura made the podium in the IA1 class (finishing 4th and 3rd), showing us that we were heading in the right direction."

Tomonori Nakamura's 2005 KX450F-SR (All-Japan Motocross Championship).
Performance of the big-bore thumper confirmed Kawasaki engineers were on the right track.

While the 450 was being developed, the motocross world had completed its transformation from 2-stroke to 4-stroke. Kawasaki's 2006 KX lineup included the new KX450F and the 3rd KX250F. Both models featured aluminium perimeter frames. The frames received minor changes as they evolved into their present-day configurations, but their basic design is the same as the steel perimeter frames Kawasaki conceived 20 years ago.
Completion of the 4-stroke lineup proved to be a big advantage for the U.S. Kawasaki Racing Team, whose racers were based on mass-production models. In 2007, the efforts of Kawasaki racers James Stewart (KX450F / SX Champion), Ryan Villopoto (KX250F / SX Lites West Champion) and Ben Townley (KX250F / SX Lites East Champion) gave Kawasaki a clean sweep of all three classes in AMA Supercross. The following year, Stewart (KX450F / MX Champion) and Villopoto (KX250F / MX Lites Champion) dominated both classes of outdoor racing. In the same period, Japanese Kawasaki racers were equally successful, with Hiroaki Arai (KX250F-SR / Team Green) taking the IA2 title in 2007 and Takeshi Katsuya (KX250F / Green Club & Junior Riders) crowned IA2 Champion in 2008. The supremacy of Kawasaki 4-stroke racers was being quickly established.

"The positive reputation we were earning through our race results was great, but not enough for the development team to rest on its laurels. Our next order of business was to get a practical form of fuel injection working. One of the weaknesses of carburettors is that fuel supply gets destabilised when landing jumps, on corner exits and going over whoops. Fuel injection solves this problem and as a bonus automatically compensates for climate variations, so changing from carburettors to FI was a natural progression."
First field tested during the 2007 season of the All-Japan Motocross Championship, fuel injection was standard equipment on the 2008 KX450F-SR. In the first round, Mizoguchi and Arai finished 1-2 in the second moto. Fuel injection was featured on the 2009 KX450F mass-production models.
"We wanted to appeal the new fuel injection to the European market as well, so in the last round of the 2008 World Grand Prix, Sébastien Pourcel (riding with GPKR at the time) competed with a fuel-injected racer. After comparing the carburetted and fuel-injected versions at a pre-race test, Pourcel chose the FI model without a moment's hesitation. In the last round he took the pole position, which gave us a sense of accomplishment, but the real drama was still to come."

Sébastien Pourcel left a big impression riding a fuel-injected KX450F-SR at the 2008 Motocross of Nations.

The stage for the dramatic finale was the Motocross of Nations event held at England's Donington Park two weeks after the last WGP round.
"Not only did Pourcel hold his own in a dead heat with America's James Stewart, his individual results put him ahead. (Pourcel went 2nd/1st vs Stewart's 1st/DNF.) They were both Kawasaki racers, so it would have been a good result no matter who won, but from an engineering perspective the fact that Pourcel's FI machine beat Stewart's carburetted racer made it an especially gratifying race. When Pourcel got past Stewart, the roar from the crowd was really incredible. They say that racing is where technology is truly tested, but that kind of emotional impression can be equally stimulating for development."
The 4-stroke chapter of Kawasaki's motocross history has only just begun. There are still plenty of blank pages for new racing achievements to fill.