Cycle News - Archive Issues - 1970's

; Zazz for the YZ125, ~ or power through the piston Figure II·A Specification "A" >n r- - Ol Figure I-B --- - - .02 1" main transfer &bO~~~Oh 1- 3 02 1' t vP· 35 ~ } OF / ~~~~~" o.. ~ .6 d ia . vent hole Note Remove all sharp corn ers or rough areas on piston . Figure I·A .024" mai n t ransfer & Note ° .- boost por±JS ' -- . c.. --- .1" .6 dia . vent ho le Rem ove all sharp corners or rough areas on piston. By des igner/tuner Arthu~ Ridderhof The Yamaha 125 Y-Z , though very refined In many areas for its medium prIce tag, still suffers from a few production cop outs. The 125 Y·Z engine is probab ly the hottest 125 in its class, but it '11 h I b fore i sti as a ong :way to go e ore .It reaches perfection, The two mam 24 prob le m areas are the cylinder and pipe, both, tough well-designed, still are not where it's at for a winning IT scramble, or MX machine. The stock cylinder If you have ever looked at the cylinder base gasket area (where the transfer ports meet the transfer cavities in the crankcase) you'd almost have to notice that Ya maha's nifty tricky 125 doesn't exactly come with this area matched In fact they are usually off by an easy 200 to 250 thousandths on the exhaust and intake facing edges . .. h o nest! Even if this is the only modification you felt like trying afte r reading this article you'd be running a horsep o wer strdnger or the equivalent o f one or two bike lengths faster at the end o f a fou rth or fifth gear straightaway . (See Figure III) The transfer to exhaust timing as in most MX engines, is mo re or less a com. promise. The Y.Z suffers from too much exhaust lead to have adequate low speed grunt for tight tracks and insufficient brute ho rsepower to p ull any sort of high gearing for the wide open tracks. The barrel is unlike most motocross bikes in th at it doesn 't have a reboreable liner. The p lated bore/aluminum cylinder might not so und a bit like the hot set-up for some of you old Lunger pilots out there. But, in perspective the plated/aluminum cylinder really is the hot set-up! The Y.Z was intended for racing not for trips back and forth to the s;ore, etc, The average purchaser will probably use his Y-Z approximately one to two hours per week, whieh comes to roughly 100 hours a year. So even if the cylinder o nly lasted 150 to . 200 h o urs, you'd be getting an awful lot o f racing in durin g that ti me . The p~rpose for the plated/alum~num bar;el IS twofold: (I) the . cy lin der weighs a b o u t half of Its c a s t iron/aluminum counterpart, and (2) the plated/aluminum barrel diss ipates heat considerably faster and bec ause of this will distort far less. The co o le r running and the more coneentric seal on the rings add up to more ponies and relatively longer engine life. , As far as I'm c:oncerned, there is.only one problem W ,lth a p lated/alummum barrel, and t~at s portmg. 1!'e chances 0 f . c h a ,:, g l n g por.t sizes and con~lguratlons and having some of the platmg flake off on a feather edge are very high . In fact, a lot higher than most shops will admit. .Beca use of this fa~t the Y-Z must be piston ported. ~at.ls, the transfer and exhaust .p ort· tl m mg m ust be chang~ d on the plS;?n ra~f,~ than on the cylmders ports. 19ure~ A a!,~ · ~ show the necessary pi ston modifications that are to take place for ITTshcramb!es and ~XII raalcers. . e piS t on WI s.o recel~e an elongated ~ole on the mtake Side to help equalize crankcase and reed area pressures as well as help the cran kc~e to ~eed. the rear boost port. .This modification helps the I?: and m";idle RPM ~or~epower Wit very little , change m high R;PM horsepowe~. . . When all the Illustrate d modifications have been completed polish the piston c r o w " to prevent carbon buildup, excessive surface drag, and promote b~tter heat ~ssipation by keeping the PISt?~ cro wn s boundary layer down to a ml.m m um . . FI~ures. II·A and B .sho w the cy l iri d er modifications that Will co m plemen t the pi;'ton modifications, an.d can be done ~thout danger of scrapmg your plated Iiner. Resonate pipe The stock pipe, while being an in teresting piece of equipment, is not really the hottest set-up around. The front cone has been designed to give a very brief but powerful low pressure wave back to the cylinder to help scavenge the fresh fuel/air char ge . The system works gre at at high speeds when you're trying to dump large quantities of gases in a very short amount of time. However, if I remember correctly, the Y·Z was intended for motocross an d a little tr actability never hurt in this form of racing. -My latest designs for the Y-Z both have a little longer low pressure wave pulse, but, either one will out-perform the stock Y-Z pipe at high RPMs . This might sound like a paradox at first, but it really isn't. Although the first cone went slightly torquer, the rest of the (Spec. " A " ) pipe went a little more on the trick side of scale, and for th e (Spec. " B" ) pipe I pulled out all the stops to squeeze out every pony the engine can handle. ( Figure I V) We chose to use an Exduser silencer for the present time, mainly to get under the legal AMA decibel limit. We felt th at with the Tunable Reed model we co uld ge t closer to the legal Db limit than probably any other silencer on the market, without going over it. Reed valve The st andard Y-Z reed valve works fine under all RPM ranges, for all you academic trivia collectors. It's the same one used with the 1972 360 MXs. There is a spacer plate available to adapt the MX 125 and 175 to use this reed, but a three-eighth pie ce of phenalic will accomplish the same task. The modified y·z port specifications will wo rk great on non- Y-Zs, but the port should be raised to th e equivalent of what would be removed from the piston cro w n. Also make the center section of the pipes 1.250 inches longer to correct for some of the internal engine superiorities th at th e Y-Z incorporates. Gea ring With the added H.P. and RPM , you'll be able to cover a lot more ground a lot faster than before. You'll find, however, for tracks with fair ly lo ng straightaways you'll need to go up a tooth or two on the countershaft, I'd suggest us ing the extra grunt power instead of the RPM which will be gained, to reach higher top speeds. The added RPM span will come in handy for those short straight passes and for tricky sections where shifting doesn 't always make it . Ignition The Y·Z ignition is definitely a fine unit. It's very hard to foul , and has a very light flywheel to keep the inertial weight way down. I haven 't experienced any problems with the C.D . dropping off at high RPMs , as with Suzukis and Kawasakis, etc. We have found in a series of field tests that the connecting rod drops off lo ng before any high speed spark dis orders arise. " Go- Fast coatings" On 0 & A Enterprises' Y-Z we completely disassembled the engine and had the rotating parts coated with Kalgards " Fa Lube" and the stationary parts with heat dissipating "Gun Kote." The results added a big plus to the horsepower situation and the change in head temperature was nothing short of dramatic. The bike was able to hold its ho rsepo wer even during the hottest days we tested it. Carhuretion Most 125 Y·Zs we have run across are jetted sligh tly on the rich side from the factory. This, if intentional, is to help keep engine temperatures down and lower the peak RPM slightly. With the increased flow of the added porting and more efficient (Spec. "A") exhaust our stock jetting on two of the three bikes we tried worked out perfectly and the remaining bike required an increase of one step on the main. With the (Spec. "A") pipe and a couple of other non MX tricks, we were required to go a "Gr ind smooth: all j oints & int e rsec t io ns Figure II-B Specification " B" Grind smooth: a ll joi nts & intersectio ns Figure III Figure IV Specif 11 .5" 11.12" t 0 1=1.55" Specif 10.5" 9 .0 1" °1 - 1 .55 " notch higher with the needle and to the next size main. Where and how to get t he goodies Both the (Spec. "A") and (Spec. " B" ) pipes will be available in "U" weld kit form in the early part of Spring, 1975. The selling price will be $35 each, and will include heade r pipe, shock mounts, etc. The (pisto n/cylinder) porting is available presently in two ways, the satin finish for $35 and the polished finish for $45. For all you o n-looking 2.

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