In 1968, Kawasaki joined Suzuki in the production off-road machines. Unlike the Suzuki TM250 that was targeted at motocross, the Kawasaki F21 was designed for the smoother scrambles tracks of America. The color brochure said, “Hit the Scrambles Trail on a Top-performer Built to Take it.” The 1968 Kawasaki F21 fuel tanks were painted a deep red with the Kawasaki aircraft company logos on both sides. In 1969, these bikes came equipped with a much brighter white and lime green tank design. The 1969 model, although not all the different from the red tank version was relabled as the F31-M Green Streak. Less than 1000 of these the F21 and F31 were produced from 1968 to 1970. It was the first Kawasaki mass produced race bike marketed for motocross, scrambles, hare scrambles and flattrack

Other than trying to tame down the power output, the 1968 and 1969 F21-F31 bikes were designed for the 250 class, but only came with a 238cc displacement and many potential buyers thought it was 250cc bike. The”238″ was not competitive against the 250 Bultacos, Ossas or even the and the Japanese Yamaha DT1 because of the displacement was not being a full 250. It was called the “Big Green Streak”, but sales wer sluggish, while its 100cc Green Streak brother was a rocketship on the track and flew off the dealer’s showroom floors.

The front forks look a lot like Italian Ceriani forks, but they Kawasaki’s own version, patterned on the popular Ceriani forks.

One of the more intriguing aspects of the 238’s engine is its rotary valve design, which comes with the ability to change the powerband with relative ease. It comes with a mild rotary valve cut-away, borrowed from the Kawasaki 175cc which snaps right into place on the 250/238. It produces very broad torque. From stock it is a simple matter to increase the valve cutaway, incrementally, and arrive at something akin to a road racer. Kawasaki rotary valves, with different cut-aways retail for $7.58.

Because of the the rarity of 1968-69 Kawasaki F21/F31 rotary valve bike they hold a veru high resale prcie on the vintage market place—often going for anywhere from $12,00 to $20,00 depending on condition.