At age seventeen, Kocinski was already a factory rider for Yamaha, in the AMA Championship Cup. He won the AMA 250 Grand Prix Championship every year from 1987 to 1989, and won the 1989 Supersport race at Daytona having started 53rd in a field of 80 riders. In 1988, he won the pole position at the 250cc US Grand Prix and finished the race in fourth place. He would also place fifth at 1988 250cc Japanese Grand Prix.
1989 was also the year of his 500cc World Championship debut. In 1990 he raced in four different championships, but the highlight was winning the 250cc World Championship in his first full season on a Team RobertsYamaha TZ250. He was a full-time 500cc racer for the next 2 years, finishing fourth and third in the championship and winning the final round in both seasons.<ref name="John... Read More