Nov 01, 2023
WE RIDE PRO CIRCUIT’S 2023 YAMAHA YZ125 RACE BIKE
The Yamaha YZ125 is uniquely appreciated among MXA’s stable of motocross bikes. At any given time, we have over 25 different dirt bikes in our warehouse of each of every motocross model from each
The Yamaha YZ125 is uniquely appreciated among MXA’s stable of motocross bikes. At any given time, we have over 25 different dirt bikes in our warehouse of each of every motocross model from each brand, and the YZ125 still stands out as a sentimental favorite. Yamaha is the only Japanese manufacturer producing two-strokes, and their YZ125 and YZ250s have aluminum frames with awesome Kayaba coil-spring forks, further differentiating them from the steel-framed and air-fork-equipped two-strokes from KTM/Husky/GasGas. Plus, with the Austrian brands ditching their carburetors and going with fuel injection on their two-strokes, the Yamaha only becomes more nostalgic.
Looking at the history of this bike, the YZ125 was all new in 2005. The only parts that didn’t get changed were the clutch plates, clutch perch, kickstarter, grips, footpegs, rear brake, hubs and rims. Of course, MXA has traditionally carbon-dated the start of the YZ125 era to 2006 because that’s when the class-leading Kayaba SSS forks were installed. In the years following the 2006 debut, the YZ125 lost its luster, and we began to moan and groan about it. KTM’s 125SX caught and passed the Yamaha, and we hoped our critical reviews would inspire Yamaha to update its engine in order to keep up; however, after years of neglect—just when we switched our tune from complaining about Yamaha’s two-stroke apathy to praising Yamaha for simply continuing to produce two-strokes and keep them at a reasonable price—they surprised us! The 2022 YZ125 was all new, undergoing its first real updates in 16 years. Yes, they kept many aspects of it the same, but that’s good! We love this chassis and suspension and really just wanted a faster engine.
As reported in our last two 125 shootouts and Yamaha YZ125 tests, the new engine didn’t live up to its hype. Yamaha, unfortunately, gave up much of the YZ125’s initial low-end snap in search of more midrange power. The higher horsepower has been better for riders who can easily access it, but it’s more difficult for the less experienced two-stroke riders whose clutch and throttle finesse aren’t so sharp. The 2021-and-prior YZ125s were easier for Novices to ride, and the KTM 125SX has always come out of the box as the better race bike. The new YZ125 engine gave up its rider-friendly low-end and sadly couldn’t match KTM’s mid and high horsepower.
Of course, it was a bummer that the YZ125 didn’t match KTM’s strength in 2022, but, surprisingly, the YZ isn’t looking so bad in 2023. Why not? The new-generation KTM and Husqvarna models took one step forward and two steps back with their new fuel-injected engines. In our 2023 MXA 125 two-stroke shootout, it was the GasGas MC125 (using last year’s carbureted KTM platform) that won the battle, with KTM, Husky and Yamaha following behind. Yes, Yamaha didn’t beat the Austrians (again), but Yamaha owners aren’t dealing with map issues, and they can make unlimited mods to their 125s, while the new, fuel-injected, KTM/Husky models are still a work in progress.
After Yamaha updated its engine and swapped out the Mikuni TMX carburetor, engine builders like Pro Circuit had to go back to work and update their YZ125 specs. Pro Circuit’s YZ125 Works pipe and R-304 Shorty silencer were reworked to meld with the 2022-23 YZ125’s needs. They also had to reconfigure the new Keihin carburetor. Compared to the prior-model Mikuni, the Keihin PWK38S PowerJet carb is more complicated. It’s known for delivering crisp throttle response with improved rev-ability, but it takes some time to figure it out. The PowerJet is electronically controlled and mapped with the CDI to provide additional fuel when the rpm is between 5000 and 9000. The PowerJet opens a separate fuel circuit that’s only used within these precise parameters to maximize power and avoid detonation. When it’s closed, the engine is only getting fuel from the main jet.
The simplest fix for 2022–’23 YZ125 owners is to switch back to the tried-and-true Mikuni carb, but Pro Circuit didn’t want to give up on the new carb and leave its customers hanging. Pro Circuit’s R&D technician, Mike “Schnikey” Tomlin, spent hours dialing in the jetting and came up with Pro Circuit’s new OEM-spec YZ125 jet kit. On our built-out YZ125 race bike, Schnikey used a 62 pilot, 165 main jet and a Yamaha Y3 needle, which can be bought through Pro Circuit, in the third clip position.
As for the combustion chamber, the cylinder and head were ported and polished by Mitch Payton, and Pro Circuit’s YZ125 “RV” piston was added. The Pro Circuit crew also added a GET ignition that was remapped to work with the modified engine and Power Jet-equipped Keihin carburetor. The special GET ignition provided Pro Circuit with the ability to fine tune two separate maps. One is named “power” and the other “smooth power.” They used Maxima Castor 925 pre-mix oil in 4 gallons of VP MRX02 race fuel, cut with 1 gallon of VP C12.
Typically, MXA runs the R-304 Shorty silencer, but on this YZ125 we opted for the ultra-cool-looking Ti-2 Kevlar R-304 silencer. What’s the difference? Internally, it’s the same, and performance-wise, it is the same. Only the material is changed. While the standard R-304 is made of stainless steel and aluminum, all the metal on the Kevlar silencer is titanium, and the shell is carbon fiber with Kevlar material woven into it, making it much lighter than the R-304.
When Pro Circuit told us that they could turn a stock 2023 YZ125 Pro Circuit into the greatest YZ125 ever, we wondered out loud how much this Yamaha YZ125 engine package would cost. The total cost came to $1473, but, of course, we couldn’t stop with just the engine upgrades. We wanted the suspension dialed in as well. Pro Circuit added over $1300 in work and materials to improve the already-impressive Kayaba SSS suspension. They added a longer Pro Circuit link arm, extending it from 145mm (stock) to 146.5mm. This stiffened up the initial linkage curve and eliminated the loose, wallowy feel of the stock setup, making the chassis more balanced. They also added the Pro Circuit Specialty fork kit, which is a technical piece that uses some Showa-style shims in the mid-valve of the Kayaba fork. Then, they added their National spring tube, which is a long tube that replaces the stock spring seat inside the forks. The stock spring seat is very short, with big oval holes for the oil to flow through, while the National tube is longer, taking up more volume and forcing the oil through smaller holes. With it, the fork has more middle-of-the-stroke hold-up and control, and it’s something that Pro Circuit even runs in its Showa race team forks.
As for the rest of the bike, we added a Pro Circuit billet clutch cover, a 100/90-19 Dunlop MX33 tire in the rear, and an MX3S on the front. Pro Circuit ran a Renthal rear sprocket with the stock 49-tooth gearing, a Twin Air filter, Throttle Syndicate graphics, and a ribbed Throttle Syndicate gripper seat cover.
So, how did it run on the track? Our first impression was that it was loud! The sweet melody of the Ti-2 Kevlar R-304 silencer echoed through the Glen Helen hills, reminding our hearts and minds of a simpler time when two-strokes ruled the roost. Since it’s a small-bore engine, the powerband was short. Especially for our Novice test riders, the YZ125 has been difficult to keep in the proper rpm range ever since Yamaha switched to the Keihin carb. Of course, for our more skilled test riders, the power window felt bigger, because they knew how to keep it in the sweet spot. The “smooth power” map (Map 2) was slightly easier to keep in the proper rpm range, but it wasn’t easy by any means; it’s still a 125 two-stroke. With this engine and CDI configuration, Map 2 had more bottom-end power, while Map 1 was more free-revving, giving the throttle a lighter feel. It was stronger but harder to maintain. Comparing the Pro Circuit bike to a stock YZ125, the power window is slightly bigger, and it’s moved higher into the rpm range, more like a factory 125 two-stroke from back in the day.
How did it handle? The YZ125 makes you feel like a hero, and with this suspension, you’ll feel even better. Don’t get us wrong, we still love the stock Kayaba settings for the majority of our test riders, but our heavier and faster guys blow through the stroke easily once the track roughens up. This chassis was balanced around the Glen Helen National track, and it held up well in the bumps for our fastest riders.
Overall, the Pro Circuit YZ125 is great for young kids who are making the transition from minibikes and are getting their feet wet on the bigger chassis. It’s equally great for serious kids who are lining up in the 125-only Schoolboy 1 and 125 B/C Open classes at AMA events and for any other rider who’s looking to improve his or her skills while having fun in a safer manner. Because of its light weight and YZ125 power, it is ultra-fun to play on and great for teaching a Novice how to carry momentum while managing the clutch and throttle. Obviously, if you’re planning to race the Pasha 150 Open classes at the World Two-Stroke or World Vet Championships, or if you’re a Vet rider looking to race in Vet classes against 450 four-strokes, then you’re better off ordering a 144cc big-bore kit from Pro Circuit instead.
\With the Yamaha YZ125 gaining a new engine last year, Pro Circuit spent lots of R&D hours developing its new YZ125 engine package before offering the finished product to the MXA wrecking crew to test.Even though there were very few bolt-on parts added to the chassis, everyone loved the look of the Pro Circuit YZ125. Throttle Syndicate hit it out of the park with these race-team-style graphics. THE GEAR: Jersey: FXR Racing Revo Pro LE, Pants: FXR Racing Revo Pro LE, Helmet: 6D ATR-2, Goggles: EKS Brand Lucid, Boots: Sidi Atojo.The Ti-2 Kevlar R-304 Shorty silencer is the same internally as an R-304, only it’s made entirely from titanium and carbon, with some Kevlar woven into it, making it much lighter.Our test riders had a blast on this bike. It’s lightweight, flickable and fast with the Pro Circuit mods inside the engine.The GET electronic control unit gave the YZ125 two map options, “power” and “smooth power.”We still miss the Mikuni TMX 38 carburetor that came on the 2021 YZ125.The GET ECU box gave Pro Circuit a wider range of tuning for the YZ125.The Pro Circuit link arm is longer than stock and it increases stability.