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/127846
silencer of the four-stroke probably also has something to do with it. Oh. you want to kn ow about the motor? Well, Yamaha hasn't told us all tha t much, though we know it has twin chain-driven cams and five valves in the cylinde r head - three for intake and two for exhaus t - and that it features w et sum p lubri ca tion. Unlike Hen ry' s works bike, the water pump is loca ted on the engine cases an d d riven off of the balance shaft ra ther than up on the cylinder head, driven by one of the cams . Also, rather tha n the twin headpipes foun d on Henry's factory machine, the production bike has a single heade r. Carbure tion for the YZ400F was pro vid ed by a 39mm Keihin FCR flat-slide, a carb uretor com monly used in road raci ng app lications. The ign ition is equipped with a throttleposition se nsor that alte rs the timin g accordi ng to the engine load an d thr ottle position. When sitting on the bike, there is little to differentiate it from any ot her YZ, save for the compression-release lever and a slightly wider rea r section , due to the increas ed s ize o f the fo ur-stroke's silencer compa red to the tw o-stroke's. Presently, th e YZ400 F weig hs 237 po unds, th ou gh ta rget weight for the prod uc tion bike is 233. The YZ250, by compariso n, tips the scale at roughly 215. Seats on all three big YZS are given a more rounded profile. Surprisingly, th e 55799 YZ400F is priced only $200 mo re than the YZ250, whic h, at $5599, is the same price as the '97 mod el. The YZ125 also comes in at the same price as last year, a t $4799. The engi ne in the ' 97 YZ250 wasn' t significantly down on power com pa red to the other bikes in the class, thou gh its power delivery felt a little lazy by comparison. So, in search of a greater "hit" ki nda feeling, the YZ250 gets a good number of changes to its powerplant. The combustion chamber is reshaped to provide a sma lle r volume a nd flatter squish band , and the port tim ing has been delayed by .5mm to enhance lowe nd and midrange response . The exhaust-po rt shape has been altered to better match the YPVS power-valve sys - . te rn, th e ti ming of whi ch ha s b ee n cha nged to help with low- to-mid range power. To enhance throttle response in low-speed corners, the primar y-drive ratio ha s been ch anged from 2.818 to .000, and friction has been red u ced in the redesigned water pump, which now eatu res ball bearings and aluminum nd steel internals ra ther than resin. lso, the left-side crankshaft and oil sea l ave a new design to red uce friction . . ounding out the changes to the powerlant are new settings for the carbu retor (Left) Unlike Doug Henry's works bike, the production bike gets a single headp ipe for the ex haust system, and the wate r pump has been moved ta the eng ine cases from the cylinder head. (Right) Yamaha's trademark five-valve Genesis des ign is used on the YZ400 s twin-cam (Above) All three ot the big YZS have significantly updated 46mm forks, which are cylinder head. (Below) The VZ400 Is eqUipped with a five- said to provide more consistent dampIng. The bottoming system has also been changed to an elastomer bumper. spee d gearbox and a manual co mpression relea se to ease start ing. (Below) Due to the four-stroke 's compression brak ing , the VZ400 uses a smaller rear brake than the new VZ two-stro kes with their updated , larger di sc an d redesigned ca liper . (Above left ) Yamaha went In sea rch of "hit" from the ir solld-but-un lnsp irlng VZ250 powerplant with a host of changes to Increase both low-speed response and top-end power. • (Above right ) Like the 250 , the VZ, 25 gets the race-team rear link and softer rear spring , and the redesigned fork that features a large r~iameter cartridge said to prov ide moreconsistent damp ing. and altered ign ition tim ing to enhance top-end hit and extend the o ve rre v capability . The YZ125, by co m p arison, pretty much ruled the powerplant department in the ei ghth-lite r cla ss la st yea r, so whil e there have been some fairly significant change s made, Yamaha has att empted to keep the same power character, but just to provide mo re of it. The combus tion chamber is slightl y changed in shape and volume, resulting in a higher compression ratio . Last year, the power-val ve tips were moved close r to the pist on for better sealing characteristics when fully closed, a nd the results were so good that Yama ha has moved them even closer for ' 98. For quicker revving, the piston and wrist pin have been ma de lighter. As on the 250, th e exhaust silencer has been made significantly longer to give better response in the lower rev range. To deal w ith the potent ial increase in power, the s hift cam in the six-speed gea rbox gets the addition of a bearing to enhance shifti ng feel, and new friction rna terial is used in the clutch. The YZ80 returns fo r ' 98 with a slightly mo re rigid fram e, with the additional gussets down the main tube near the cradle, and differen t front brake-pad material. Price is $3149. A co ntingen cy program will be in effect for the 1998 season. The YZ125 sho uld be in dealer showrooms by July, while the 250 should sta rt arriving in August. to' 7