What killed the two stroke in motocross?

The beginning of the end for the relationship between two-strokes and AMA Pro Motocross and Supercross came in the form of the Yamaha YZ400F. Introduced for the 1997 Supercross series, the four-stroke Yammie was a one-off build Yamaha executives had talked AMA into accepting on the premise of environmental safety. AMA agreed that if the company mass-produced the motorcycle, it could indeed enter its bike into the pro circuit.

The YZ400F debuted on May 17, 1997, at the final race of the season in Las Vegas. Those who had heard about the bike doubted its efficacy, as two-strokes were simply the answer in motocross at the time. But Rider Doug Henry opened a two-second gap in the first lap and struck the first nail into the coffin of the two-stroke in motocross. The heavier engine stabilized the bike, delivered superior torque, and exposed the weaknesses of the zippy two-strokes. Motocross never looked back.

AMA ultimately allowed two-strokes and four-strokes to compete against each other, with one major caveat: Two-strokes needed to have half of the displacement. In other words, two-stroke 125cc bikes would have to race 250cc four-strokes, and 250cc two-strokes against 500 cc four-strokes. The die was cast. After a few years of mixed-class racing, it was no more — the day of the two-stroke was over. If you tune into an AMA Motocross series today, you'll see nothing but 250cc four-strokes and 450cc four-strokes. The day of the two-stroke was over.

The future of the two stroke in motocross

The good news is that two strokes are alive and well despite their absence from the pro-circuit. Cheaper and relatively easy to maintain, amateurs and weekend warriors are still zipping their way around dirt tracks all across the country. Two-strokes command a cult following, and their distinctive buzzy exhaust note is looked upon fondly by old-timers who remember the days of yore and young riders who maintain their own bikes. Wealthy factory teams also don't need to worry much about the higher maintenance cost. In fact, there is still a call for the return of the two-stroke.