Boyesen 640 Power Reed For: Yamaha IT250 all models; Yamaha IT400 1979-1985; Yamaha IT425 1979-1985; Yamaha IT465 1979-1985; Yamaha IT490 1979-1985 For: Yamaha WR250 1989; Yamaha YZ250 1978-1982; Yamaha YZ250 1989; Yamaha YZ400 1979; Yamaha YZ465 all models; Yamaha YZ490 1982-1983 The top reed is lightweight and resilient for crisp throttle response at partial throttle or low RPMs; the stiffer, bottom reed is ported to provide maximum flow and horsepower at higher RPMs Longer overall life: Boyesen power reeds outlast all other aftermarket reed petals Power reeds use a patented dual stage fiberglass design for more air flow, more air velocity and more power; minor change gives major horsepower improvements and fuel economy