Cycle News - Archive Issues - 1980's

• ••••• CN 00 0') ..... , engineersdeveloped a unique output,combustion efficiency and,wonderofwonders, gas centrifugal-governor system Inaddition to its ability to mileage. accurately track enginerpm, Of course, the machines it has two otherveryimportant that featureYPVS (the YZ125 thingsgoingforit ReIiability. and YZ250l alsofeature our Anddurability. unique, through-frame waterIt works like this: The cooling system So all that engine's crankshaft speed is newfound power willstay transmitted tothe centrifugal- with you from the starting govemor As the engine speeds gate to the checkers. YPVS and water cooling, ~,the~~ moves a rod-and-lever linkage a combination guaranteedto which rotatesthe va1ve upkeep the YZ125 and VZ250 ward, out of the exhaust port. right wherethey've always And asenginespeed debeen. creases, the valve moves back At the head oftheir class. down intothe port. In the throttleopening charts. youcan see theincredi· ble effect of YPVS. Engine outputat low-speed is much Boththe YZ125andfZ250 greater withYPVS than withpotoerplants are uater-cooled out it,no matterwhat the so all that newfound power throttle opening. Andengine outputat high-speed (where a two-stroke motocross engine uxm't fade aioay: reachesits peakperformance) is not decreased at all. WIth YPVS, exhaust port timingat low speeds is retarded, increasing the compression ratio,Y during et, actualrunningtests, troubles normally associated with high compression ratios such as piston or ring seizure or . scuffing on the cylinder wall didn'toccur because ofthe low heat load at low speeds. While inthe highspeedrange, nearthe maximumtorqueand As enginespeed increases , power area wherethe heat thegovernor mechanism loadis highest. exhaust port moves a rod-and-lever . timing advances so the comlinkage whichrotatesthe pression ratio doesn't rise to valve upward, outofthe A highexhaustportmakesfor The heartofYPVSconsists the dangerpoint exhaust port. And as the good high-rpm horsepower; oftwo basicmechanisms: Allthat,and you don't engine speed decreases, but not-so-good low-rpm The valve itself . haveto lift a finger the valvemoves back down performance. Andthe valve control , YPVS is completely intothe port. For years,engine designers mechanisms. automatic. Just twist the have testedandexperimented Both mechanisms are throttle and it consistently with systems that would not incorporated intothe cylinder gives you the best ofboth worlds.Plenty of torque at JlAU" THlI0771.EOpeN1VG FUll THRCfTTLE OPENING "low-rpm and plenty ofspeed at high-rpm, All the while improving overall power 1PVSis driven by a unique centrifugal-governor system. Besides beingable to ~ accuratelytrack engine rpm, thissystem is also extremely reliable and durable. . I ---i--------...:.J_J •• ••• THE • INTRODUCING GREATEST IMPROVEMENT TO THE TWO-STROKE ENGINE SINCE THE TWO-STROKE E GINE. , As any racing mechanic will tellyou, the two-stroke engine is almost perfect for motocross. Lightweight, cornpact,easy tocool and very powerful. Now aboutthe "almost:" QUARTER THlI0771.EOPEN1VG "'" ~ L/ ' 10 - '"' ,-' ~., / -oOIlIl ~, -,J _ _ _ "...rns Ever since 1881 when Du gald Clerk invented it, the two-stroke engine has suffered from one severly limiting design factor; Exhaust porttiming, Set by the height ofthe exhaust port,exhaust port timing determines the nature ofthe engine's power A low exhaust port heightprovides excellent low-rpm power; but high-rpmperformancesuffers. 28 j V/ ~ , / .,' '.4" ",' . '. ... ""'5 only change exhaust port timingin response to variations in engine speed.but insurereliability under extremely high exhaust port temperatures. As you might have guessed. Yamaha succeeded. , The new Yamaha Power V System (YPVS for alve short) variesthe exhaust port timingin response to variations in engine speed. It'sbeen tested ina wide varietyof racing machines and has proven not only togreatly improve engineoutput, but alsoto decrease fuel consumption. as you can see in the drawing on the right The valve is shaped to fit ascloseto the cylinder bore as possible.The center portion ofthe valve is cut to match the shape of the exhaust port and, when rotated, to raise and lower the height ofthe exhaust port, changing the timing. To control and drive YPVS, ....-_+-..,. The centerportion of the power valve iscutto match theshape oftheexhaus! port, and, when rotated, toraise j -......_ , and lowerthe height oftheexhaust port, ~~~~~~~~ changing the timing. Yamaha The Yamaha PmaerValve System variestheexhaust port timing in response to variationsin enginespeed. ,-

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