If you’re looking for a super-fun two-stroke that will reward you with a lot of fun and an unforgettable exhaust note, you almost can’t go wrong with a Yamaha YZ125 project bike.
Depending on what you’re after, YZ125s from just about any year can make fun projects, whether they are from the vintage, evolution or modern era. Naturally, older models aren’t going to be competitive in modern classes, but the beauty of the modern YZ125 is that there are loads of them out there, and parts are easy to find. The perfect case in point is this Yamaha YZ125 project bike, which was built by Jay Clark Enterprises in collaboration with two-stroke tuning wizard Tom Morgan of Tom Morgan Racing. Clark had been looking for a cheap YZ125 project bike for one of his typical hyper builds for a while when he came across the one you see here.
“Yamaha YZ125s are great bikes and have been the same basically since 2005,” Clark said. “One of the downsides to our sport for new riders is that the price of maintaining a modern four-stroke has continued to rise. YZ125s and YZ250s are kind of a two-edged sword in that they are fun, less expensive and less complicated to maintain, but there has been little in the way of development or improvement in their engine packages for a long time. If four-strokes had not taken over how much better would our two-strokes be now?”
Call it a protest of sorts, but Clark wanted to see just how well he could make an YZ125 project bike run by using parts from the sources that he works with on most of his rolling showcase dirtbikes. After some shopping, he came across a 2011 YZ125 that seemed to be available at the bargain price of $900. Believe it or not, this particular YZ125 project bike was intended to be an economical build, but after getting it home and taking it apart, that plan went out the window.
“The cylinder was in bad need of full repair, and the bottom end was no better,” Clark said. “So, I just thought, ‘Why not go all out and build a full on race ripper motor?’”
To do give this particular YZ125 project bike a big power boost, Clark enlisted the help of Morgan, who has been handling engine work for everyone from factory teams to top racers for decades. Morgan, who began tearing into engines when he was just 4 years old, gained a lot of experience that took him all the way to a position with Kawasaki’s American-based R&D team in the 1980s, and he would go on to play a vital role in winning five national championships with riders such as Jeff Ward and Mike LaRocco, earning five AMA Mechanic of the Year awards in the process.
After shipping the engine off to Morgan, the two decided to get a little stealthy and actually bump Clark’s YZ125 project bike engine displacement from 125cc to 134cc. It’s a small bump, but it’s worth it, Clark says.
“The YZ134 is totally stealth,” Clark said. “The bike still sounds like a 125 but produces more mid-to-top horsepower while maintaining a normal YZ125-style powerband. The added power really works on longer, faster tracks.”
Getting the engine to displace 134cc isn’t all that complicated, but it does take some shop work. The stock YZ125 bore size is retained, but the crankshaft stroke is lengthened. The cases must also be modified to ensure the longer crank throw has enough clearance inside the crankcase. To make everything happy, Morgan used a balanced crankshaft from Crank Works, Inc., which adds the right amount of Mallory metal to the crank in order to reduce vibration while also altering the crank pin placement to increase the stroke. Morgan also used a complete bottom end kit from Hot Rods, which includes a new rod and main bearing seal kit as well as a complete engine gasket set. The piston is part of a Vertex Pro Replica piston kit, which includes the ring, pin and clips, but Morgan modifies the piston to add a boost port of the correct open duration for increased performance.
The piston rides in a Nikasil cylinder that has been completely reconditioned by Millennium Technologies. While Millennium Technologies prepared the cylinder, Clark had the option of boosting displacement even more by boring the cylinder, something he chose not to do on this particular build. However, going from 134cc to 144cc is a matter of boring the cylinder to 58mm to yield a total displacement of 144cc with the stroker crankshaft mod.
Morgan completed the machine work on the engine by changing the cylinder port timing and head dome shape to take full advantage of the stroked crankshaft. Bolt-on mods include a MotoTassinari V-Force reed setup. Morgan also dialed-in the carburetor jetting by using a 6CHY5-80 needle placed in the middle position, a 420 main jet and 35 pilot jet; the carburetor rebuild kit is from All Balls Racing, and the carburetor draws fresh air from a UniFilter Two-Stage air filter tucked into the air box. On the exhaust side, an FMF Fatty expansion chamber and FMF Shorty muffler are used to increase power and improve throttle response.
While Morgan buttoned up the YZ125 project bike engine, Clark worked on setting up the rolling chassis. Right off the bat, he opted to modernize the look of the YZ125 by swapping the plastic to a Cycra Racing YZ plastic kit that mimics the look of the 2015 and up YZ125. A Moto Seat Cool Custom seat cover and DeCal Works Semi-Custom graphics kit with pre-printed number plates backgrounds give the bike a personalized and factory-fresh appearance.
For better durability, ICW fortified the YZ’s radiators by installing its signature bracing. The radiator is topped off with CV4 formed silicone hoses and a high-pressure radiator cap. Durability is also the reason for using a complete Hinson clutch kit basket, inner hub, pressure plate, fiber plates and steel plates along with a Hinson clutch spring kit.
When it came time to refurbish the controls, All Balls Racing also supplied a throttle cable, clutch cable and brake master rebuild kits. The handlebars are Renthal 604 Fat Bars while the grips are Renthal Tacky grips with a Motion Pro Titan throttle tube under the right-side grip. Scar titanium footpegs are used to add even more trickery but also to give the rider a more solid footing on the bike.
To complete the chassis rebuild, Clark opted for an Applied Racing triple clamp, Factory Connection-modified suspension, and also used a Pivot Works steering stem bearing kit, swingarm kit and linkage rebuild kit. Bolt Motorcycle Hardware was also used where needed. The Dunlop MX 3S-shod wheel set is from Faster USA, and it is completed with a Tusk 270mm oversized front rotor and a Tusk rear rotor, Tusk front and rear steel braided brake lines and Tusk brake and sprocket bolts. The brake pads are from Renthal. Clark also replaced the stock sprockets with a Supersprox 13-tooth front sprocket and Supersprox Stealth 48-tooth rear sprocket.
Other parts include a complement of Works Connection bits, including a Works Connection Elite clutch perch, brake caps, front brake lever and rotating brake handlebar mount. TMDesignworks supplied the chain guide and chain block, while Bolt Motorcycle Hardware chain adjuster blocks.
Clark invited us to Cahuilla Creek MX Park to spin some laps aboard his YZ125 project bike, and if you ever cut your teeth on a 125cc two-stroke, then we don’t have to tell you just how much fun we had fanning the clutch and keeping the Tom Morgan Racing-built engine screaming at the top of its lungs while negotiating the long, fast and sandy main track.
Riding a well-tuned 125 is a beautiful thing. This particular machine exhibited excellent throttle response and made clean power once we got it singing. That said, you won’t mistake it for a 200cc or 250cc machine. Momentum conservation is still the key to fast lap times, and you will find yourself furiously rowing the gearbox in an effort to accelerate out of corners and over jumps. We can’t say we expected more from what is effectively a 10cc boost in displacement, so we weren’t disappointed. The TMR motor simply feels like a really strong 125, with a nice mid-range pull that snaps into a zippy top-end. Fall off the pipe, however, and you still have to downshift to regain the lost drive. It was also no surprise, however, that the YZ125’s Factory Connection suspension delivered a cushy ride, handling chop and ruts without upsetting the chassis while also resisting bottoming over big jumps.
But is all the work worth it? That depends on who you are. Clark is willing to admit that he went overboard on this particular build…shocker! Then again, if you like 125s, a bike like this one is about as much old-school fun as you can have in your riding gear.
Jay Clark Enterprises/Tom Morgan Racing Yamaha YZ125 Project Bike Build Info
Your Privacy Choices