Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/127204
1990 Yamaha YZ motocrossers and YZ2S0WR Big, little changes for new YZs Wh en Yam aha 's fleet of 1990 YZ m oto cr osser s 'a n d th e YZ250WR debu t on showroom floors in Se p tem ber, yo u 'll fin d sign ificant cha nges toth e YZI25 and 250, as well as the YZ250WR. The YZ80 and 490, on the ot her hand, were left unchanged excep t for som e m inor cosmetic su rge ry. Both' the YZ125 and 250 motocrossers look simi lar io the 1989 mod els, except for sli ghtly di fferentshaped radiator shro uds, new graphics o n' th e seat, the removal of th e red border a rou nd th e side panel number pl at es a nd red (ins tead of go ld) an odized upper fork tubes. The color scheme on all four YZs are the sam e with wh ite pl astic a nd wh itepainted frames, red seats a nd silverpa inted engi nes. Ev en th e red colored YZ logo o n th e radia tor shrouds ar e unchanged. ... The 1990 YZ250's motor _ features a shorter connecting rod, porting changes, as well as a new boost port between .the intake and left rear transfer port, and reed-valve modifications. Both the YZ250 and 125 retain 19-inch rear wheels. ~ The YZ125, as well as t he ...... 250, featu res a m uch low er center of gravity and significant modifica t ions t o t he upside down front forks. Ya maha designed the 125's motor for more m id- and top-end power. _ The YZ250WR, which made it s T ' debut last year, is back aga in . It f eatu res t he same changes as t he YZ250 MXer. but it now has an 1 8-inch rea r wheel. It ret ai ns it s wh ite and blue color scheme; the YZs are still white and red . The real changes are in tern al in both the 125 and 250 YZS. The 125 has a new flat -crown piston, th e cylinder head volume is sligh tly reduced, and th ere are also porting cha nges to th e cylinde r's transfer ports. In the YPVS pow ervalve system , a torsion spring and stopper ar e added between the YPVS linkage and valve to absorb linkage flu ctuations which, according to Yamaha, provides more accurate valve co ntrol. The valves center diameter is incr eased O.5mm to 21.5mm to boost midrange power. A new exhaust pipe is used to enhance th e porting and power-valve changes. The crankshaft diameter is slightly larger to increase inertia mass, and is rebalanced to reduce engine vibration. Chassis-wise the ,YZI25 and 250 me new features such as tuned-up upside down forks , more pow erful and bigger front and rear di sc brakes, rear shock linkage cha nges, longer swi ngarms, an d overa ll low er cente r of gravity. The forks on both bik es are radicall y cha nged. Not o nly are th e fork slide rs now red-anodized, but the y are now sligh tly lar ger in diam eter . The spring rates ha ve been in creased and revised damper cartri dges are used for better separation of air and oil. The new fork s are also more adjustable than befor e with 20-way external compression and rebound adj usters , as well as spring preload adjusters. Other fork ch a n ges include stronger dust seals and a beefier stanch io n tube plastic protectors, similar to the on es found on the 1989 and '90 Honda CRs. The rear shock on both bikes are . revalved. The motor in the 250 features a