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/127949
NEW BIKES 1999 Yamaha YZs he v d o n't look much d iffere nt , bu t Yamaha w ill tell yo u other• wise. Wh ile the 1999 YZ400 and WR400Fs featu re relati vely few changes for '99 - a fte r all, th e fir st run of ' 98s were just released a few mon ths ag o the new two-stroke 125 and 250cc YZS feature man y refinement s and some significa nt chan ges. The YZSO, on the oth er hand , rem ain s unchanged for 1999. The 1998 Yamaha YZ12 5 wa s th e und ispu ted king of the class, and Yamaha d id not want to rest on its lau rels and le t the competition ca tch up. In stead, Yamaha made many "s mall" changes, and Yamah a claims some of these modifications make th e YZ125 eve n fas ter tha n before. Starting 'w it h th e e ng ine , th e '99 YZ125 has new porting specs. All of the exhau st ports have been en larged by l rnrn , a s have th e s ca ve ng ing ports, which have also been reshaped . Sma ll changes were also mad e to the po w er va lve's va lve h old er a nd g o ve rno r, including a larger-diameter go vernor ball . The YZ 's TM X36 ca r b u re to r h a s changed, as well. Whil e it' s still a 36mm Mikuni, the overall len gth (top to bottom ) and width (fron t to back ) of the carbu reto r bod y has been sig nificantly reduced, and the throttle valv e itself has a different shape. Yam ah a claims these changes im prove low - to midran ge performance. A n d , to rou nd off th e e ng ine cha nges, a new exha u st pi pe has been in corpora ted . Th e p ip e a lso fea tu re s s li g h t ly t hinn er w all s, dow n f ro m 1.0mm to 0.9mm. A new wo rks-type (a la Kawasak i) tail cap has been added to the end tip of the aluminu m silencer. In th e susp ension d epa rtm ent, th e s w inga r m he ad h a s been s lig h tly resh aped , a n d th e relay-a rm (bo ttom linkage) mounting position is also sligh tly changed - as is the linkage itself, which is longer. (Eve n though th e relay-arm mou nting position has been changed , the overall ratio is the same as before.) The sealing structure of the rear a rm -pivot section has also been cha nged from an outer- type seal to an inner-type seal. Yamaha claims that the new swinga rm is also lighter overall than last yea r's. Th e re va lved rea r s hock is 15m m longer (fro m mou nting hole to mou ntin g ho le ), a n d capacity h a s been increased . Yamaha claims the YZl25 has im proved "so ft ini tial acti vation , soft intermediate stroke, with no bottomingout contact." Yam aha co m p letely redesigned th e rear ax le, which is now 2mm larger in diameter and ha s gone to a two-pla ne ty p e of hea d ins tead of a hexa go na l shape, which means yo u don ' t need a second w re nch to keep th e en tire ax le from sp inning when loosening or tightening the nut. Minor weight-sa ving cha nges wer e mad e to th e YZ's rea r hub, includ in g going from s teel to a lu min u m s poke nip ples, laced in to new Takasago /Excel rims. As a result of the hub changes, the driven sprocket sees a new mounting pitch, which basically means older YZ rear sprockets won 't fit on the '99 YZS. Up front, the taper of the outer fork tu b es ha s b een changed to imp rove st rength, and Yam aha claims the forks T 32