GP’s Classic Steel #58: 1976 Yamaha YZ125-X

For this week’s GP’s Classic Steel we are going to take a look back at the 1976 Yamaha YZ125-X.

For this week’s GP’s Classic Steel we are going to take a look back at the 1976 Yamaha YZ125-X.

In the seventies, Yamaha actually produced two race bikes for the 125 class. The MX125 was less expensive, mellower and aimed at the novice and entry-level market, while the YZ was made for serious racers. In 1976, the Yamaha YZ125 was the most high-tech 125 money could buy.

In 1969, Yamaha made its first attempt at a 125cc “motocross” racing machine with their AT1-M scrambler. The AT1-M cost $539 and was little more than an “enduro” bike fitted with knobby tires and stripped of its lights. By all accounts, the AT1-M was a terrible bike and an inauspicious beginning for Yamaha. After several more years of lackluster 125 machines, Yamaha finally stepped up to the plate with a serious racing effort in 1974.  The all-new YZ125 featured a high tech motor and went head-to-head with Honda’s new 125 Elsinore. This first generation YZ, while quite and improvement over its MX125 cousin, was not up to unseating the mighty CR125M. The Honda won on the track and in the showrooms, and outsold the YZ three to one.

In 1976, Yamaha made several refinements to its 56 x 50mm two stroke mill.  Highlighting the changes were an all-new cylinder, expansion chamber and carburetor. The carb in particular was a massive change, as Yamaha ditched the C models puny 30mm Mikuni in favor of a massive (for the time) 34mm mixer.

For 1975, Yamaha upped the ante with its ’75 ½ YZ125-C (Yamaha did make a limited number of YZ125-B’s in ’75. They were basically ’74 YZ-A’s painted yellow and quickly phased out mid-year). The new YZ-C would see the introduction of Yamaha’s revolutionary “monoshock” rear suspension system. While not perfect, the new mono was a huge improvement over the previous YZ’s short travel shocks and helped propel the YZ to the top of the class in 1975.

The YZ125-X’s two-stoke mill pumped out a little over 20 horsepower from its 123cc’s. That was just shy of the Honda and Suzuki, but far better than the outdated Kawasaki. On the track, its advanced reed-valve intake gave it a wide powerband (for a mid-seventies 125) that was far easier to manage than the piston-port Honda. In the end, it dominated the anemic Kawasaki and pippey Honda, but was not quite up to unseating the awesome RM125-A for top motor of ’76.

Also making its debut in ’75, would be Suzuki’s replacement for its unloved TM125. The all-new RM125 would feature long travel suspension front and rear and combine it with an easy-to-ride motor. The new RM’s laid down dual shocks actually outperformed the high tech monoshock of the Yamaha, but its mellow five-speed mill held it back from claiming the top step of the podium.

By far, the most visually unique feature on the YZ125-X were these reservoirs, or  “accumulators” mounted on top of the Yamaha’s Kayaba air forks. These air chambers contained the high pressure portion of the fork’s dual-air chamber suspension system. Because of their odd appearance and resemblance to a street bike instrument cluster, they became known as the “tachometer” forks.

By the time the new ’76 models would make their debut, the 125 class had become the hottest one in American motocross. After several years of being discounted as a “beginner” class, the tiddlers were turning out to be the most popular on the track and in the showrooms. The arrival of the CR125M Elsinore in ’74 had exploded interest in the eighth-liter division, and the Japanese were quick to step in to meet demand.  By ’76, the 125 class was an all-out war for the dollars of America’s youth.

In the mid-seventies, the 125 class was the most hotly contested division in motocross.

Because of this amazing level of competition, both the RM and YZ would see major revisions for ’76. After less than a year on the market, both bikes would receive new chassis’, revised motors and beefed up suspension. For Honda, the increased competition would mean a slide back in the rankings from class King to also ran nearly overnight. After decimating the competition in ’74, the Elsinore would loose out to the superior performance of the YZ and RM in ’75. For ’76, the outlook would get even bleaker for Honda and Kawasaki. As Suzuki and Yamaha pushed the envelope of design, Kawasaki and Honda seemed content to send out mildly tarted up ’74 models yet once again. With Honda and Kawasaki apparently stuck in neutral, 1976 looked to be a two horse race for 125cc supremacy.

What is old, is new again.  While most people think air forks are some kind of radical new idea, in truth, they have been in use for decades.  The 35mm Kayaba air forks used on the ’76 YZ employed two air chambers, in place of the traditional springs found in most forks. The lower chamber controlled the first few inches of travel, while the high pressure chamber located in the accumulators handled the rest. They were extremely adjustable, but difficult to tune for the inexperienced. Once sorted, they were the best forks available in ’76.

While at first glance the YZ125-C and YZ125-X appeared very similar, they were actually very different bikes.  For ’76, Yamaha stretched the wheelbase a full inch and raised the seat height nearly two inches. Both the monoshock and new “air” forks punched out more travel than the ’75 and offered improved performance. In the motor department, Yamaha spec’d out a totally new cylinder, revised the ignition and bored out the carb for more performance. The YZ125-X may have looked like a freshened ’75, but it was actually a whole different animal.

By far, the most noticeable addition to the ’76 YZ was its new Kayaba air fork (and you thought your new CRF450R was cool-Yamaha had it in ’76!). These units have gone down in history as some of the oddest ever to be fitted to a motocross machine due to the two large reservoirs that were affixed to the top of its 35mm tubes. The two “accumulators” served as high-pressure air reservoirs, with each leg actually containing two separate air chambers. The lower pressure system in the lower stanchions handled the first few inches of travel, while the high-pressure chambers in the accumulators handled the rest. By tinkering with the air pressure in these two chambers and fine-tuning oil level, the air forks offered a huge range of adjustability.

Stripped of all its bodywork, one can see how dominated the YZ chassis is by its monoshock rear suspension system. Early on, this innovative design was a big improvement over conventional dual shock arrangements. By the early eighties, however, its high center of gravity and lack of a variable rate linkage would doom it to the history books.

In terms of performance, the Yamaha’s new air forks smoked the rest of the field. They punched out an excellent 7.68 inches (nearly an inch more than the previous year), which was top of the 125 class in ‘76.  Action was very smooth and they gobbled up rough tracks with abandon. Testers raved about their amazing bump absorption and some even labeled them “perfect”. By mid-seventies standards, these were excellent forks.

Changes to the rear suspension were not nearly as radical as those to the forks on the YZ-X. Yamaha’s patented monoshock had made its debut only 8 months earlier on the YZ125-C and was kept more or less intact on the X model. Modifications for ’76 included an increase in travel of nearly an inch to 7 ¾ inches and an additional 10% of air and oil volume in its “De Carbon” shock. In addition to the longer shock, a new longer swingarm (by and inch) was also spec’d to aid stability and fight the unwanted wheelies that were an issue on the ’75 model.

For ’76, Yamaha bumped up the travel on their patented Monocross rear suspension system to nearly 8 inches.  This kept it competitive with the new RM125-A and outpaced the Honda and Kawasaki. In terms of performance, the Yamaha’s mono was excellent at taking big hits, but choppy over small impacts. Once again, it was Suzuki who stole the accolades for having the best rear suspension of ’76.

Performance wise, the rear of the Yamaha was the second best performer in ’76. It was good at absorbing big hits and well set up for faster riders, but too stiff on small impacts. It still exhibited a bit of the  “Yama-hop” kicking under breaking that had plagued it in ’75 and required more skill to set up than the others. In terms of ranking, it lost out to the ultra-plush Suzuki, but handily beat the crappy Honda and abysmal Kawasaki.

While suspension is always important in motocross, in the 125 class, there is only one thing that truly matters: Horsepower. That was true in ’76 and it is still true today. When you have very little of something, every bit counts. The interesting thing about that fact, is how very differently all the manufacturers went about making that power in ’76.  The CR, YZ, RM and KX all used unique motor designs that year, in the search for the ultimate motocross power plant.

In 1975, Factory Yamaha rider Tim Hart took the new YZ125 to second in the 125 standings. One year later, a little known kid from the deserts of California would blow across the nation like a “Hurricane” on his works Yamaha 125 and rewrite the history books.

In the case of the Elsinore, Honda had stuck with the solid and proven piston-skirt-controlled port arrangement for its littlest CR. This “piston port” arrangement meant that the intake charge was wholly controlled by the position of the piston skirt as it moved up and down inside the cylinder. The advantages of this design were simplicity, reliability and an unobstructed intake. It had no additional parts to manufacture or fail and tended to have excellent flow characteristics at high RPM’s. It was a proven design that had been employed on two-strokes for decades and was known for providing strong top-end power, at the expense of low-end performance. Without the benefit of the rotary or reed check-valve to prevent the intake charge from “bleeding off” as the piston moved through its stroke, the piston-port design was inherently difficult to tune and provided a narrow spread of power.

In 1976, all four of the major Japanese manufacturers used very different motor designs for their 125’s. Honda went with a simple and inexpensive straight-through “piston-port” design (far left). Yamaha went the high tech route with their fancy “reed-valve” configuration (middle), and Kawasaki stuck to its tried and true “rotary-valve” (far right) for the little KX.

At the other end of the complexity scale, you had the Kawasaki, which used a “rotary-valve” to control the intake charge on its little KX. The basic premise behind a rotary valve is that it uses a spinning disc with holes machined into it to control the flow of air and fuel into the motor (hence the commonly used nickname for the design “baloney slicer”). This disc is mounted directly to the side of the crankcase and allows very precise control of the intake timing. It is far better at providing a wide powerband than the piston-port arrangement, but tends to be complex and bulky do to the need to mount the intake and carb on the side of the crankcase.

Yamaha’s iconic “Bumble Bee” motif made its debut in 1975 (at least in America, in other markets around the world the YZ’s were white and red) on the new YZ’s. Prior to that, racing Yamaha’s wore a serious silver and red color scheme.

With the YZ, Yamaha took a more conventional approach by adding a reed-valve to the traditional piston-port style intake. The idea behind a reed-valve was to install a one-way check-valve to the intake tract of a two-stroke. Unlike a four-stroke, which has the benefit of a defined intake and exhaust cycle, a two-stroke must somewhat comingle these duties. With the piston-port intake, it is easy for some of the intake charge to escape out of the crankcase in the split second between TDC and the closing of the port. The advantage of a reed-valve is that it does not care what the piston is doing. A reed-valve opens only when there is more pressure on the upstream (carburetor) side than the downstream. Because of this, the motor can be tuned to make the best use of the incoming intake charge. With a reed-valve motor, an engineer is freed from having the piston do both the intake and exhaust ports jobs and can better tune the engine for optimum power characteristics. The downside of the reed-valve arrangement, was that it somewhat restricted flow into the motor (particularly in ’76, before the advent of lightweight fiber and dual-stage reed valves).

Of the four systems used in’76, by far the most interesting was the Suzuki “Power Reed-Valve” set up. It combined the straight through intake of the CR (good for top-end power) and the reed-valve intake of the YZ (good for low end response) into a semi-case-reed hybrid. While this novel design would eventually be supplanted by superior full case-reed intakes, in 1976, it provided the best spread of power available in the 125 class.

Of the four motor designs used in ’76, perhaps the most interesting was Suzuki, who tried to go the Goldilocks approach. Suzuki apparently looked at both the old school piston port approach and the new school reed-valve design and thought “hell, why not just use both”? The result was a unique “semi-case-reed” design that Suzuki named ‘Power Reed-Valve”. The PRV arrangement actually used two separate intakes; one with a reed-valve and one without. The idea was to use the straight-through piston controlled intake to provide unobstructed flow at high RPM and supplement that with a small two pedal reed-valve to charge the crankcase at low RPM.  It was a novel approach, but was it the best? Read on to find out.

One issue on early Yamaha Mono’s was shock fade. With the shock buried deep inside the bodywork, there was little opportunity for cooling air to reach the unit. Later editions would enlarge the reservoir and move it from under the rear fender to the front frame down tube where it could receive the maximum airflow.

Much like the suspension wars, the battle for powerband supremacy came down to two real contenders in ’76. Only the Yamaha and the Suzuki offered the horsepower and usability to be King. On paper, the Honda looked like a contender, pumping out 21.53 HP (most of the group), but conversely, it also pumped out the least torque of the four. The little CR was a screamer, with all of its thrust pumped out over a very narrow range. Off the line, it was an endless sea of bog, before cleaning out and revving to the moon. It was the classic two-stroke “all-or-nothing” powerband and by far the hardest of the four to ride.  In the Kawasaki’s case, it was a lack of ponies, plain and simple that did the KX in. The KX’s rotary valve mill put out a pleasant and easy-to-use spread of power, but at 18 HP, it gave up over three horsepower to the other three 125’s.

Even thought they offered excellent performance and tuneability for their time, the added cost and complexity of the Yamaha air forks would mean they would only last one year. By 1977, the YZ’s would be back to conventional spring forks.

That left only the Yamaha and Suzuki to fight it out for the top spot, a crown the RM would claim by virtue of its superior power spread. In the case of the YZ, Yamaha fitted a very large (for the time) 34mm mixer on the YZ that focused most of its power higher up in the powerband than on the RM. Its powerband was wider than the pippey Honda, but not as torquey as the flexible RM mill. In terms of outright power, the top three were all within a half of a horsepower of each other on the dyno, but on the track, the superior midrange torque of the RM gave it the edge. When you added in the Yamaha’s recalcitrant and notchy transmission, the Suzuki roosted away to a convincing victory.

While no XR75 in terms of sound output, the YZ125-X offered the largest silencer and least objectionable exhaust note of the ’76 125’s.  BBRRAAPPPP!

One area where the YZ took a step back from ’75 was in weight. In the switch from the C to X model, the YZ gained a whopping 12 pounds! That was no small amount of weight on a bike that only puts out 10 foot pounds of torque and made the YZ the heaviest of the Japanese 125’s. Also not working in the YZ’s favor, was its high price relative to the other machines. At $1027, it was the most expensive up front, and required additional special tools to service and tune the exotic air suspension. Another concern for neophyte Yamaha pilots was the complexity of tuning those trick forks. Because they offered a huge range of adjustability, they also offered a huge opportunity to screw them up. More than a few YZ pilots spent countless hours chasing their tail in search of the ultimate set-up in ’76.

At $1027, the YZ125-X was the most expensive of the Japanese 125’s in ’76. It offered state-of-the-art suspension and a powerful reed-valve motor for the rider skilled enough to make the most of its potential.

The YZ125-X was a very good race bike in 1976. It offered the state-of-the-art in motor and suspension technology for the rider who could maximize its potential. It was easier to ride than the top end only Honda, but not as flexible as the excellent RM. For the average 125 pilot, the RM offered an easier- to-ride mill, plusher shocks and easier tuning. If, however, you were a fast pro, the high tech YZ was hard to beat.