Building A YZ125X Off-Road Project Bike

DBT YZ125X Project Bike

Story by Trevor Hunter; Photos by Trevor Hunter/Craig Hunter

After riding our YZ125 on the tracks for a few months and giving you guys an update, we decided to do something a little different with our small bore YZ. With no major redesigns over the last decade, building a little more unique YZ125 is tougher but we were able to do so by going off the beaten path. With no YZ125X on the market, we decided to do a small build on our own focusing on making our YZ125 capable of enduring the tough conditions of off-road riding along with a couple performance upgrades as well.

Durable yet capable accurately depicts our “YZ125X” build.

After our last update, we installed a new top OEM Yamaha Piston Kit after putting 20+ hours on the original top end. Could we have waited longer? Yes. However, two-stroke maintenance is very simple and easy and we would have needed to pull the cylinder off to replace the rings at a minimum so we opted to do a full rebuild while we were in there. Plus, working on a two stroke is so simple compared to modern day four-strokes that it’s kind of fun!

Suspension worked well in all conditons wit just a few minor adjustments to the stock settings.

Suspension wise, we ended up sticking with stock suspension as it worked well enough for high speed/GP conditions typical to the West Coast for our 155lb tester. Suspension on a 125cc is always a little less critical as its lack of power and weight make it pretty easy to setup and after some fine tuning, we got the YZ working pretty good in a variety of conditions. We ran the sag at 105mm for a good neutral setting that worked well everywhere. The clickers on the forks were at 9 out for the compression and 15 out for the rebound while the low-speed compression on the shock was at 10 out and the rebound at 12 out. Depending on the trail conditions, we would change a click or two but we found this to be a very good base setting that could be ridden anywhere. It even worked better than some modified suspension we have ran in the past!

Our ZipTy Racing IMS 3.2 Gallon Tank was fueled by 91 pump gas and Maxima 927 premix.

To start, we installed a Zip Ty Racing 3.2 Gallon Oversized Tank by IMS. A pure necessity, installing an oversized gas tank is always the first step when going off-road and the 3.2 gallon tank increases fuel capacity by one gallon while keeping the ergonomics and feel of the bike very close to stock. In addition to the tank, IMS sent us their CORE MX Footpegs for added grip and the added durability they add; although, the stock Yamaha footpegs are some of the best in the class with its wide platform and superior grip. Additionally, the bike ran on Maxima 927 for the premix and MTL XL gear oil. Maxima Oil is some of the best in the business and we have been running it for years in personal bikes for both riding and racing.  

Bullet Proof Designs provided the protection to ensure our bike could endure the tough off-road conditions.

Bullet Proof Designs sent us a set of Radiator Guards and a Rear Disc Guard for some added protection. A simple tip over or one rock is all it takes to take out a radiator and the Bullet Proof Designs Radiator Guards offer a lot of protection with a little weight gains. Similarly, the Rear Disc Guard is an all-in-one aluminum piece replacing the stock backing plate adding more protection with very minimal weight gains. I’ve beat up quite a few rear discs over the years on my own YZ’s making this piece a little extra important for my riding.

Boyeson reeds and a DT-1 Air Filter handled the intake department for this bike.

Next on the list was a Boyeson RAD Valve and Power Wing. Two-strokes respond to different reed cages and we enlisted Boyeson products for the better throttle response and added torque along with a more lively motor. We ended up making a jetting change leaning out the main jet from a stock 430 to a 420 and dropped the needle one clip to the 2nd position. With the Boyeson  installed and stock jetting, the bike ran a little rich and would load up quite a bit when riding in slower technical singletrack making the bike less than pleasant to ride. After a few minutes of singletrack, we would have to clean the bike out often. However, with the jetting changes in place, the 125cc two smoker ripped in every condition we could find here on the West Coast. To keep the bike breathing easy, we installed a DT-1 Filters Air Filter and Filter Cage with its SuperSeal Base providing a safe and secure fit to prevent sucking dirt and sand into the motor.

A Mika Metals Hybrid rear sprocket and Chain paired with a TM Designworks Factory Edition SX Slide-N-Guide Kit made sure we had maximum drive in every condition.

To put the power to the ground, we utilized a Mika Metals Hybrid Sprocket and Pro Series Chain and aligned it with a TM Designworks Factory Edition SX Slide-N-Guide Kit. The Mika Hybrid Sprocket has an aluminum core with a steel outer ring for a lightweight yet durable setup – perfect for our 125 Off-Road Build. Additionally, we have ran TM Designworks Slide-N-Guide Kits in the past and their durability and fit is exceptional while also adding a little style to the bike with some blue accented pieces. Additionally, we installed a TM Designworks Enduro Coverage Skid Plate to protect the frame and vital engine cases. The plastic skid plate is very durable and sleek while still guarding the all-important water pump and covers. For tires, Maxxis is a big supporter of off-road racing and riding sponsoring major series’ like Big6 and GNCC and the Maxxis Maxxcross SI Tires hit the dirt as their durability has proven to be some of the best in our past testing and racing.

A GPR Stabilizer and Fasst Co. Flexx Handlebars made up the cockpit along with ASV F6 Levers.

A GPR Stabilizer was installed to handle stability at high speeds after having years of experience running this setup. The GPR differs from other steering stabilizers like the Scotts system in that it has constant damping while the Scotts dampens as the handlebars turn away from center while having no damping turning back to center. They both have their pros and cons and some riders prefer one over the other based on the constant damping vs. half damping. After using GPR Stabilizers for years, I have grown to like how it reacts and am more familiar with its characteristics leading me to using the GPR system on our “YZ125X” build.

Reflex Racing Handguards are fairly newer with an updated design from what we have tested in the past.

In the controls department, we installed one of our favorite handlebars – Fasst Co. Flexx Handlebars – for the  feel and added comfort needed in the tough off-road conditions. With the sub-mounted GPR Stabilizer and my lack of height, I elect to a run a 12-degree Moto Low bend as the standard moto bend is a bit too high. In the past, even riders around the 6’0 mark prefer the lower bend as well as it keeps the bar height a little closer to stock. Additionally, AM’E Half Waffle Grips are the preferred grip of choice for myself due to the super soft compound and feel of the grip. Likewise, we installed a complete ASV F4 Lever Set. The F4 Series is ASV’s lower end model; however, I prefer the feel of these forged aluminum levers with its more rounded lever. It comes complete with a Pro Perch featuring an on-the-fly adjustable wheel. Finally, we installed Reflex Racing Reflex 1.0 Handguards to protect our hands from the rigors of off-road. The Reflex handguards are a full wraparound system with their patented “Cable Union System” allowing the handguards to flex in the event of a crash or contact with debris avoiding breakage or bending.

From sand to rocks, our YZ125X handled it all like a champ!

With just a few add-ons, our YZ125X is a potent off-road race bike without breaking the bank. Yamaha’s tried-and-true YZ125 is a very capable machine in stock trim and doesn’t need much to be an off-road bike. With some necessary protection upgrades like a skid plate and handguards along with an oversized tank, the YZ is ready to rip the trails and make it back to the truck in one piece worry-free. Also, with the addition of the Boyeson RAD Valve and Power Wing, the motor is very effective and appealing to a variety of different riding conditions from tight singletrack to fast sand tracks. To top it all off and possibly the most important aspect in an off-road bike, Yamaha’s are known for their durability and we found the YZ125 to be virtually bulletproof once again for 2018.

Ride along around a sand track aboard the DBT YZ125X with Trevor Hunter:

Ride along around a local desert track aboard the “YZ125X” with Trevor Hunter: