Cycle News - Archive Issues - 1970's

Yamaha's trials secret By Lane Campbell Not satisfied wi th the original monoshock trials prototype, the factory has already sent a second generation machine to Mick And rews in England. The new bik e go t it s bap tism at the English round on March 16 and carried Mick to th ird place in a cliff-hanger m eet t hat Andrews led but briefly. The British pap er s all took p ictures of the little rubber bootie ove r the super-tri ck carbu ret o r and promptly c h r i s t e ne d i t " Ya m a ha 's Se cr et Weapon. " The bi g secret is a new pumper ca rb, fi tted to be tter mat ch carb ure ti on to the reed va lve at ultra-low engin e speeds and allow use of a slightly bigger (2 8m m) venturi. Major reason for th e switch would appear to be th e fa ct that at pion king speeds, airflow in the inle t tra ct is no longer stead y-s ta te but a series of distin ct pulses. Sin ce a normal carburetor depends on airflow through the venturi to cr eat e "draw " to suck fuel out of the float bowl in metered amoun ts, this can also lead to unsteady fu el flow . In practice , wh at this means is a series of foa m y gushes from the idle air bleed, not necessarily a correct mixture, nor ne cessarily co m p le tely vaporized. This is a com mon problem with all trials eng in es, however, Mick Andrews and Yamaha have taken a new ta ck in solv in g it. The pumper is co n t ro lled by a bleed line from th e cran kcase (stan dar d practice ) and m ay actuall y perform as a borderline positive-p ressure injector a t lo w speeds . Sin ce a reed val ve is by nat ure a "demand" valve , it can be assum ed th at the strength of pressure pulsations in the cr an kc ase arc a direct measure: of t he en gin e' s air d em and. By couplin g c ra n kcase demand to th e pump d iaphragm , th e carb be comes a cru de form of fu e l i njec tion th at is (t heo re tical ly) more sensitive to engi ne dem and th an a no rm al carb uret o r a t extreme lo w spe eds . T he desired end result is be tter low -en d th rottle res ponse and free do m from lo ad ing up. Appare ntly, it works. Something else t ha t works very we ll is th e can t ilevered m on o sh ock rear suspension. Hear t o f the syste m is a Belgian-designed gas/oil sh oc k un it similar to , but ligh te r th an , th e units fi t te d t o la st year ' s fa ct ory motocrossers. Weight shaving on the un it was cr itica l, since it mo un ts relatively high in the fram e, and every pound in thi s area measurably affects th e mach in e 's top-hamper. S in ce tr ials co m petition generat es less severe and less freq uent heavy shock loads than motocross , ab out four pounds wer e saved without co m promising th e unit's reliab ility. While weight at the top is critical , the 1974 works bike is ac tu ally 2 0 or more pounds heavier overall than the 197 3 version. This was at Mick's req uest, as he ha d indicated at the Yamah a t ria ls school earlier this year. A ccording to Andrews, the 165 ·1 7 U pound weight of his earlier prototype would be fin e for a 125 cc, but under the thrust of a 250cc or larger engine (which is transmitted thro ugh the swingar m virtually undamped by the suspension) th e whole motor cycle will tend to jump abou t on rough steep cli mbs. He said at that ti me he'd like for his 1974 mount to weigh 190 to 19 5 pounds, and it is right on target. Going along with Andrews' theory about machine behaviour under thrust, th e rear susp en sio n must work at least as well as the fro nt suspen sion in order to cope. Total travel is over seven in ches. Eyeballing the photos with a scale, you can begin to figu re th e effective sp ring rate. Note first that with M i ck's muscular I7(}.180 pounds aboard, th e lower member of the can ti lever swingarm run s parallel to the ground, as it should. Unloaded, th e suspen sion has dropped a good 2 ~ to 3 inc hes. We'll assume t ha t under thrust , th e rider's weight is concentrated o n th e f ootpegs. Sca ling footpeg/wheelbase relationship, that says ro ughly 70% of Mick's weight (125 pounds) pre-loads th e suspension 2 ~ to 3 in ch es , or tha t the effective spri ng rate, measu red at th e rear ax le, is on th e o rder of 45 pounds/inch , es timat ed . This is roughl y eq uivalen t t o fitting 3 2 pound springs on each side with co nven ti onal shoc ks . It's qu it e soft , an d t he e xtreme so ft ne ss goes hand in glo ve wit h th e extreme am o unt of travel p ro vid ed. All t hose who saw th e seco nd gene ra tio n m ach in e a t th e ro ck y Eng lish Round in Wale s reported that th e rea r end wo rk ed very we ll, especially in the cli mbi ng sections . It wo uld seem that Mick 's wis h had been gran te d . The producti on TV· 25 0 with oi l injec tion is about I ~ inch es wid er in the cases th an a Sh erpa T . On Mick's works bike , th e o il injector has been removed, a flatter righ t side case fitted , and a full three inches shaved off the case width . St eve Graham, o ut of Southern Cali fornia, is riding with th e narrowed cases, but th e general availability of the trick parts is still a big question mark. The other big 'qu esti on : when, if ever will th e YZT250 become a production model ? . Japan's oil crisis clouds everybody's crystal ball at t he moment. Best guess is that an y and a ll features of the Mick Andrews' prototype (in cludi ng the pumpe r carb and so me form of the cantilever suspension) wh ich can be I reproduc ed in m ass production will be cran ked into the cu st o m er line on a p iece meal b asis, timed as economic reality dictates. It will certainly no t be Calendar Year 1974, probably not during 1975, maybe by 1976. For those American trials riders who are beginning to approach international stature, it's a package worth waiting for. • The " secre t " carb (ab ove) w it hout its ru b be r boot ie . Setscrews are likely for in-field adjustment of h igh and lo w speed jetting. Below. lo wer mount for monoshock can be seen just below rear of expansion chamber. Cantilever swi ng arm is a we ll·triangulated truss. Specifications (Dimens ions estimated by scal ing photos) Wheelbase Saddle Height - unloaded Ground Clearance - unloaded - loaded Steering trail Footpeg Height - un loaded - loaded Footpeg-to -Rear A xle el l ....: unl oaded - loaded Overall weight (est.) 53 inches 31 inches 12% inches 9-10 inches 5% inches 14 inches 11· 12 inches 14 inches 14%-15 inches 190 pounds 27

• Cover