2018 KTM 125 SX Project Bike Riding Impression

Riding a 125cc two-stroke is some of the most fun you can have on a dirt bike.

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.

Riding Jay Clark’s 2018 KTM 125 SX project bike at Cahuilla Creek MX in Anza, California.Spencer Owens

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.”

KTM’s 125cc two-stroke dirt bikes are some of the best in the business, so Clark was pumped to pick up this used 2018 model. It wasn’t taken the best care of, but Clark is a pro at bringing a bike back to life.Spencer Owens

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.

Although the 125 SX featured a few upgrades Clark wanted, it still needed some repairs and tender loving care.Spencer Owens

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.

The bottom end was deemed usable, but farther up in the engine needed some attention. Clark sent the cylinder and head to Tom Morgan Racing (TMR) for porting and modifications. Because the cylinder had a minor damaged area, Millennium Technologies was called upon to repair and replate it.Spencer Owens

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.

WP’s air fork has come a long way and while it was very good in 2018, there are still plenty of riders who prefer the feel of a coil-spring fork, including the prior owner, who had Factory Connection’s WP AER 48 single coil-spring conversion kit installed. Clark was happy to have it and just asked Factory Connection to revalve the fork and shock.Spencer Owens

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.

Hold it wide, feather the clutch, and smile. Clark’s modified 125 SX ran supremely clean from idle to high rpm with the aftermarket Keihin PWK 38mm Air Striker short body carburetor and jet kit from JD Jetting installed, and was easy to keep in the meat of the power by 125cc two-stroke standards.Spencer Owens

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.

In addition to being well-balanced and having plenty of bottoming resistance, the Factory Connection-massaged suspension did a remarkable job of soaking up more minor impacts, even with the rider sitting while landing to flat from a roller.Spencer Owens

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.

The 125cc two-stroke dirt bike is the gold standard for learning proper riding technique, such as maintaining momentum, as well as having a blast on a motocross track.Spencer Owens

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.

Gearbox

Helmet: Arai VX-Pro4

Parts List

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

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