Cycle News - Archive Issues - 1990's

j. --------------------------------------~-------~~~~ have less stiction tha n befo re. Cha nges have also been m ad e to the fo r ks' valves . To im prove clearance, the footpeg brackets have been shortened by almost 3m m so that the footpegs d on' t stick ou t as far. Pr etty mu ch all of th e suspensio n an d chassis chan ges tha t we re made to the YZ]25 were also made to the YZ250, but th e sa me ca n no t b e sa id for the motor, w hich, for all int en ts and purposes, is all new . You nam e it, and it's been changed . Th e YZ250 now features a longers tro ke mo tor w it h a 66 .4m m x 72mm bore- and-stroke con figuration , as co mpared to la s t yea r's 6Sm m x 6S.Sm m nearly squ a re-bo re co nfigu ra tio n . Th e result of these changes, Yam aha claims, is an im p roved to rq ue b and and im p ro ved low- a nd midspeed performanc e. Ya ma ha also ch a nged th e ent ire mounting posi tion of the engine, resulting in the low ering of the crank center position down ward som e ]3 mm, and a reduced forw ard tilting of the cylinder fro m ] 0.7 degrees to ju st 2.5 degrees . These cha nges are clai med to im pro ve the YZ250's overall center of gra vity. Th e size of the cy li n der head ha s bee n slightly red uced , both in combustion volu me and overall ou tside diameter of the head . The height of the spa rkplug seati ng su rface, however, has been raised slightly, from 33.] to 33.7mm. Both inta ke a nd ex ha ust po rts have new s hapes, and po rt timing has a lso been a ltered . Major changes were also mad e to the now-thr ee-valv e YPVS exhaust power valve sys te m . Th e YPVS val v e is chang ed as well , ha ving go ne from a rotary type to a slide type. And to top thin gs off, Yamaha joined the power-jet ca rb u reto r bandwagon . The power jet is con trolled by a solenoi d va lve (r u n off t he ig n it io n s ystem ) mounted to the Mikuni carbu retor. The CPU of the COl unit has bee n changed from a fou r-bit to an eight-bit sys tem. In th e t ransmission d epa rtm e n t, ma jor cha nges were m a d e as we ll. Yama ha basically rep laced the en tire gea rbox with the stronger and slic kershifting un it used in the YZ400F. Also, the clutch cable is ro u ted through the left engi ne case (next to thecl utch cover), and the clutch cover is n ow sealed by a n O-ring instead of a gas ket. Like th e ]25, th e YZ250 's ex ha ust pipe is new and featu res thinner walls. The silencer also fea tures the new tail cap. Information regard ing the two fourstrokes, the YZ400 and WR400F, is still a little ske tchy, but we d o know that they wi ll feature - like the two-strokes - the new s w inga r m , lin k a g e, Takasago / Excel rims, lighter hu bs and sp ro cket. They also get larg er 240mm rear dis cs. Yamaha isn' t read y to release infe rrna li on reg a rd in g th e 40 0' s eng ines, thoug h we d on't beli eve th ere w ill be major changes. The YZSO and ]25 are both schedu led to be released in Aug ust, the YZ250 in Sep tem ber, a nd both the 400s in O ctober, and no ne of the motorcycles will see l~ any price increases. 33

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