Cycle News - Archive Issues - 1980's

s i n e teen e ig h ty -n i ne is rapidl y approaching a nd a new YZ250 is upon us - the YZ25o-W. T o the eye, th e W model looks iden tical to the 1988 Ll-model, but looks are abou t the o n ly thing the two motorycles have in common. We ma naged to ge t our hands o n the first 1989 production - not a preprod uc tion, n ot a prototype YZ250W th at arrived into the co u n try; the sa me m od el that is j ust no w h itt ing th e showroom floors. T he most o bvio us new feature o n th e YZ is th e Kayab a upside do wn , or invert ed, fork. It' s what Yamaha ca lis a " fo urt h ge ne ra tio n" fork, because it 's g on e throu gh four co m p letely different design cha nges si nce Yam ah a began testin g th e invert ed fron t sus pe ns io n a few years ago. T he Kayab a fork features exte rna l co mpress io n damping adj us tme nts, and the sa me typ e or cartridge design rou nd in co nvent io na l forks. Wh eel travel is 12 inches. T he in vert ed design has m an y adva ntages - ri g id it y is o ne of them. T h e sta nc hio n lube d iam eter is actua lly smaller (4 I m m) th an modern co n vent io na l Iorks (sliders on the bottom a nd the sta nchion tu bes on the top), but si nce th ere is more o verla p (sta nc hion tubes ins ide th e sliders whe n co mpressed ) wi th th e in verted design, th ere is very littl e Hex, a nd less Ilex m ea ns m ore sta bili ty a nd co ntrol whe n ja m mi ng o ver g narly gro u nd, and m o re precise steeri ng in th e corners. Plus. since th e bottom tubes no longer project belo w th e fron t ax le, th ere is no chance o r ca tchi ng ruts while leaned o ver in turns: Sin ce th ere is littl e fork flex more strain is put on th e frame, so th e fram e's s tre ss points h av e been N st rengthene d. A sq uare-sectio n front down tube is now u sed , a n d th e fram e-cradle tubes are la rger with a m a ssi vel y buil t- u p box-sect ion wh ere the sw ingarm pivots. The rear sub-frame is now deta ch able and lo wer in the back section , giving the sea t a fla tter layo u t. The Iootpegs are a lso mou nted slightly farth er ba ck . Along wi th the new fram e a nd fork is new rak e a nd trail measurements. .T h e trai l has been increased by l m m to 120mm, and th e rake has bee n red uce d by o ne deg ree to 27 degrees. No t o n ly is th e frame a nd fork stronger, so is the sw ingarm . It 's noti cabl y larger a nd now i ncl udes more reliable push-type cha in adj usters, instead or th e pull-typ e. Attac hed to th e sw in garm is a new chai n g uide, whi ch features a lu min u m side pl at es a nd rep laceab le p las tic inserts. T he chai n ro ller pl asti c material is cha nged to im prove durabili ty a nd reduce Yam aha 's fa mous chai n sla p no rse. A 19-in ch rear wheel is now round o n the YZ instead or th e standard 18in ch whee l. Even th ough th e rim d ia m eter is o ne- inch larger, the outs ide tire d ia m eter is un ch angedl T his is accomplished by in sta ll in g a lo wer profile tire, In th eory, by keepi ng the outside tire di ameter th e same as th e outside diameter of a n 18-in ch tire. th ere is less sidewall give, or bu ckl ing , when poweri ng out or tu rns, a nd this is supposed to improve steering control a nd traction . But w ith th e ad va ntages comes th e di sad vantages. Si nce the re is less sidewa ll , th e innert ub e is more lik el y to ge t pinched and cause flat s th an a n 18-inch tire, especia lly o n roc ky terrain. And, until th e 19-in ch rea r wheel ca tch es o n in th e indu str y, tir e selecti on is going to be limited. Both rear an d Iront tires are Dunlops. The 1989 Yamaha YZ250 is hightlighted by an upside down fork , a 19T o improve braking power , both front a n d rear disc brak es have doubl e-pad ca lipe rs, and th e cooli ng slots have been elo nga ted to im p rove cooling and p ad lire. Other chassis cha nges in clude new Mi ck y Dym ond-b end h andl eb ars, which are flatter a nd straigh ter than last yea r's Iorearrn-cram pers. The n ew -styl e ba rs a re n o w rubb erm ounted. The m o tor, too, go t a good dose o r chan ges. A new cylin de r is used wi th a sma ller reed bl ock to increase gas- Flow speed at low rpm a n d im p ro ve lo w-speed throttl e response. A six-petal reed -valve is u sed ins tea d o r a n eig ht-peta l. P o rt in g h a s b een a ltered to en ha nce bo th top-end a nd bottomend response. The YPVS power-va lve has timing changes. The va lve o pe n s 250 rpm la ter. starting at 6000 rpm , a nd rnaxes o u t a t 6750 rpm . Plugs in th e crank web s have been added to in crea se primary compressio n , which in turn is supposed to make power o u tp u t m ore sn appy a nd reduce vibration. T he Yamaha no w co me s with a flat -slide M ik uni 38m m T M ca rb uretor. whi ch repla ces th e o ld-s tyle round-sli de design. A larger a irbo x is used , as well as a three-tim es larger ru bber boot between the carbu retor a nd the airbox to improve a irflow. Gas it! Before we threw a leg over th e new . YZ, a Yamaha spokesman said, " T he difference between th e '88 and '89 (YZ models) is as dramatic as th e difference between th e '87 and '88." Much to o u r su rp rise, he was right. The new YZ sim p ly has better power output a nd smoothe r su spension ac tio n th an th e I 988'model, and th e flatt er se a ti n g position and straigh ter h andlebars improves th e Yamaha 's rid ing posti on. The bi ggest change is suspens io n, a nd th at mainl y has to do with th e fork. So far, th e Kayaba fork o n th e Yamaha is th e best in verte d fork we've ever ridden - m odified or stock. It absorbs th e ha rd impacts with ease, especially land ing off big jumps, a nd still mainta ins th e ac tion to handl e the sma ller bu m p s witho u t feeling sti ff, Bu t the fo rk's best characteris tic is han d li ng th e slam mers, the rea l gnarly pou nd ing th a t normal y send shock wav es up the rider's arms. The Ya ma h a h a ndl es th e h ard-hi tt ers silky smooth w ith no bo tto mi ng . The o n ly hint or shake or uneasiness emerg ing from the Iront end became evide n t ov er the smaller bumps, usu all y at hi gh speed , on th e bra kes , a n d sligh tly leaned ov er approach in g a turn. Sometimes th e fro n t en d woul d twitch , requ iring a ti ghter re-a ssuran ce g ri p o n th e handlebars, but th at 's a bo u t a ll.

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