Video By: Spencer Owens
After nearly disappearing from motocross tracks in the mid-2000s with the advent of modern-day four-strokes, two-stroke dirt bikes have made a comeback in a large way in recent years. Whether it’s the lower maintenance and repair costs in comparison to their valve- and cam-equipped counterparts, how easy they are to work on, or simply the fun factor and enjoyment that comes from riding them, consumers are voting with their wallets and opting to purchase two-stroke off-road motorcycles, whether they be new off the showroom floor or pre-owned and possibly in need of some TLC.
Buying used and sometimes neglected bikes is not only part of Jay Clark’s job, but something he enjoys. As a longtime project bike builder and having been involved in the motorcycle industry for decades, Clark has a vast amount of knowledge and experience working on dirt bikes that helps him spot a diamond in the rough when scouring the classified ads in search of the subject for his next build—the latest being a 2018 KTM 125 SX, which we tested at Cahuilla Creek MX in Anza, California.
“This bike was bought for a good deal, and it had been raced by some amateur kids and was not taken the best care of,” Clark explained. “The frame paint is about the only thing we didn’t do [to it]. [We’ll] save that for another time,” he continued with a smile. “The bottom end seemed good, so we just got the cylinder and head sent off to Tom Morgan Racing (TMR) for some modifications. The cylinder ended up having a small damaged spot that we needed Millennium Technologies to repair. We greased up the bearings and changed out what was needed while it was down, and had the suspension serviced by Factory Connection, who already had a single-spring conversion done [to the fork] to get away from the stock WP AER 48 setup.”
Having not ridden a 125 in quite some time, it took me a few laps to get my shift points down to where I was using the power most effectively. One standout quality of Clark’s 125 SX was that it pulled third gear around the sweeping outside turns of the Cahuilla Creek MX vet track with ease so long as you put in a little clutch work. The Brembo hydraulic clutch on KTM’s entire full-size dirt bike lineup is something to behold on all of the models, especially on a small-bore two-stroke like the 125 SX where you are using it a lot, mainly when negotiating inside lines and exiting corners. The Brembo unit provides a smooth yet definitive pull and resists fading regardless of how much you use it.
I was also impressed with how long the bike pulled in each gear before having to upshift because, although you are shifting a lot to keep the bike in the meat of the powerband, I could ride second and third gear for a notable amount of time by 125cc two-stroke standards, which helped prevent a time-consuming upshift on the shorter straightaways of the track before entering another turn. Clark had the JD Jetting kit installed in the Keihin 38mm carburetor dialed, and the bike ran supremely clean from idle to high rpm.
Since 2017, the KTM 125 SX has come with a WP air fork—what was then called the “AER 48” and is now referred to as an “Xact” fork. Despite the name change, the overall design of the Austrian air fork has largely stayed the same, though WP has taken the already-good performance it had when it was introduced and made it even better since by continually modifying the internals. Despite that, many still prefer the feel of a coil-spring fork, including Clark. Luckily, Factory Connection’s WP AER 48 single coil-spring conversion kit was already installed when he bought the bike, so all he needed from Factory Connection was a revalve.
Although the suspension was sprung and valved on the stiffer side for me, noticeable traits still emerged from the Factory Connection setup. It was well-balanced and had plenty of bottoming resistance. Although there weren’t many braking or acceleration bumps on our test day, some square-edged holes on the backside of a few jump landings were present, which the suspension absorbed well and did a superb job of preventing any unwanted feedback to my hands and feet. One specific trait I noticed was how well the shock soaked up a particular landing to flat from an average-sized roller. I was sitting down upon landing to get as much traction to the rear wheel as possible. Normally, such an impact would provide a jolt through my body, namely my back, but such was not the case as the shock absorbed it completely.
The KTM chassis is another part of the Austrian manufacturer’s motorcycles that I especially appreciate. They are light, nimble, handle predictably, and have some of the best ergonomics due to how well the bike is laid out. The 2018 125 SX is especially a featherweight with a claimed weight of 192 pounds without fuel, which is detectable the moment you lift it off the stand and when throwing it around on the track. Being that KTM narrowed the radiator shroud area on the 2019 to current models, the 2018 bike feels a little bit bulkier between the knees when sitting by comparison, but not excessively. Also, the ’18 machine’s less rigid frame than the latest-generation bikes’ helps provide some additional rider comfort, especially in rougher areas of the track.
When all was said and done, I had an even bigger smile on my face at the end of our day of riding than normal. The reason being that after spending the entire summer testing the new 250 and 450 four-stroke motocross bike offerings from each manufacturer and rounding them up for shootouts, spending a day riding a two-stroke—and a small-bore one at that—really reminds you of how enjoyable it is to hold a bike wide open, enjoy the sweet smell of premix, and focus solely on having fun—which is the main reason we all get into riding motorcycles in the first place.
We would like to extend a big thank-you to Jay Clark for all of his help this past year for providing us with Dunlop tires to use for our dyno runs and motocross shootouts, and of course for giving us opportunities to ride his awesome project bikes. We are already looking forward to riding his next build and are thankful we were able to end the year of testing on a high note at the controls of his superb 2018 KTM 125 SX.
Helmet: Arai VX-Pro4
Goggle: Oakley Airbrake MX
Neck Brace: Alpinestars BNS Tech-2
Jersey: Alpinestars Supertech
Gloves: Alpinestars Techstar
Pants: Alpinestars Supertech
Boots: Alpinestars Tech 10
Vertex Pistons
Full piston kit (piston, rings, wrist pin, and circlips)
Tom Morgan Racing
Cylinder porting and head modification
JD Jetting
Keihin PWK 38mm Air Striker short body carburetor
Jet kit
Millennium Technologies
Repair and replate cylinder
FMF Racing
Factory Fatty pipe
Shorty silencer
Cometic Gasket
Top-end gaskets
Supersprox
Front sprocket (13-tooth)
Aluminum rear sprocket (49-tooth)
Motocross chain
Pivot Works
Steering stem bearing kit
Linkage bearings
Dunlop Tires
Geomax AT81 front, 80/100-21
Geomax MX12 rear, 100/90-19
All Balls Racing
Throttle cable
SRT Off-Road
Complete wheelset
Hinson Racing Clutch Components
Clutch plate kit
Billetproof clutch cover
Uni Filter
Two-stage air filter
Moto Tassinari
VForce4R reed valve system
**UFO Plastic**
Full plastic kit
Mud flap
Factory Connection
Suspension revalve and setup
WP AER 48 single coil-spring fork conversion kit
MotoSeat
Seat cover
Works Connection
Front and rear brake caps
Rotating bar clamp
Hourmeter and mount
Steering stem nut
Elite axle block kit
Valve stem caps
Factory II stand
DeCal Works
Semi-custom graphics kit
Preprinted number plate backgrounds
Graphic guards
ICW Radiators
Radiator straightening and bracing
Fasst Company
Rear brake clevis
Rear brake return spring
TM Designworks
Rear chain guide
VP Racing Fuels
C12 fuel
ODI Grips
Podium Flight MX handlebar (Champ bend)
MX V2 Lock-On grips
Tusk Off-Road
Front oversize rotor
Rear rotor
Front and rear brake pads
Klotz Synthetic Lubricants
R-50 two-stroke premix oil
Scar Racing
Titanium footpegs
Bullet Proof Designs
Front brake lever
Clutch lever
Acerbis
Skid plate