MXA RACE TEST: THE REAL TEST OF THE 2024 KTM 125SX TWO-STROKE

Q: FIRST AND FOREMOST, IS THE 2024 KTM 125SX BETTER THAN THE 2023 MODEL?

A: No. We expected to see some updates to the mapping, but the only updates came in the suspension department, and they are so minor that we wouldn’t have known about them unless they told us. The compression shim stack inside the forks has some minor changes to make the mid-valve damping feel more fluid and comfortable. The shock also has new high-speed compression damping settings with a smaller gap per adjustment. We couldn’t feel a difference on the track.

Q: HOW DOES FUEL INJECTION WORK ON THE 2024 KTM 125SX ENGINE?

A: The KTM 125SX engine was all new last year, introducing fuel injection to the motocross range for the first time. However, it’s not the first time that KTM has fuel-injected their two-strokes. The enduro-specific two-strokes from KTM and Husqvarna have had Transfer Port Injection (TPI) since 2018. The TPI off-road bikes were oil-injected, so the oil and fuel were mixed for you on the bike. It was a weird concept for two-strokers to pour straight 91-octane pump fuel in their tank and then fill up a separate oil canister and let the bike handle the process of mixing the two. 

Thankfully, the TBI version of KTM’s fuel injection is back to the same old 40:1 pre-mix ratio that we know and love. TBI stands for “Throttle Body Injection” and it uses a throttle position sensor (TPS) and dual injectors to create the proper fuel/air mixture. As for the injectors, one is positioned before the throttle flap and one behind it, creating the optimal amount of spray.

The new KTM/Husky/GasGas two-stroke engines also use a gear-driven electronic power valve, creating the opportunity for two different maps that can easily be switched on the track. This isn’t the first electric power valve to be used in motocross, but it’s unique because it’s controlled via the ECU, not by engine rpm like other power valves. Because of that, KTM can create different power characters and multiple map options by tuning how quickly the power valve opens with the ECU. However, with a small-bore 125cc, you don’t need a “mellow map,” you need all the power you can get. The KTM 250/300 big bikes use the power valve to create multiple map options, but the KTM 125SX mapping is based on fuel and ignition timing. The 125 mapping is simply set for “aggressive” or “full power” all the time and the map options are just to make the bike richer or leaner. The rich (green) map is intended to be used if you’re riding in deep sand and/or adding extra stress to the engine or hearing some detonation, while the standard (white) map, is made for everywhere else.

 Q: HOW DOES THE NEW 125SX ENGINE RUN ON THE TRACK?

A: Although the 125SX was all-new in 2023, the engine is still very much built off the powerhouse 125SX engine KTM had in 2022 and earlier. The gearbox and clutch are the same still, the port timings and compression ratio are unchanged, while the water jacket was redesigned with a new thermostat. KTM’s goal for the new engine is that you shift earlier and make use of the power in the mid-range, it’s not meant for over-revving like a carbureted 125.  

On the track, the new fuel-injected engine created a smooth and friendly roll-on power that was easier for novice-level riders to manage, it didn’t have as much on/off switch-like power that 125 two-strokes are traditionally known for. It got into the power without hesitation and with less clutch action required. The carbureted KTM 125SX of 2022 and prior was known for its more advanced style of power that required more skill to use properly. For 2023 and 2024, KTM’s combination of fuel injection and an electronic power valve for the 125SX is helpful off the bottom. 

Through the midrange, the 125SX is fast, and it pulls hard on top, but beware, the rev limiter will catch you off guard! We’re not used to having a rev limiter on a 125 or any two-stroke for that matter, but it’s there. You have to be quick with shifting, because it doesn’t like to be ridden on the limiter. 

Q: HOW POWERFUL IS THE 2024 KTM 125SX ENGINE ON THE DYNO?

A: It’s not as powerful as the carbureted 2022 KTM 125SX. The fuel-injected engine is slightly slower off the crack of the throttle. At 7500 rpm, the new and old KTM engines are the same until 10,800 rpm, where the 2022 KTM 125SX continues to climb, and the fuel-injected KTM 125SX starts to taper off. The 2024 KTM 125SX peaks at 37.2 horsepower at 11,200 rpm, while the 2022 KTM 125SX peaked at 38.2 at 11,400 rpm.

Q: HOW DOES THE 2024 KTM 125SX HANDLE?

A: KTM indeed uses the same frame for all of its big bikes, two-stroke and four-stroke; however, KTM uses a different frame cradle on their two-strokes to accommodate the smaller engine. Still, the rest of the frame is the same, from the forged gussets on the front of the frame to the rest of the frame’s geometry. One nice aspect about the 125SX is that it doesn’t take the full 10 hours of break-in like the KTM 450SXF. Multiple factors contribute to this. Of course, the 125SX engine is smaller, lighter and less powerful, but it also comes with softer suspension valving as well, which makes for a more plush ride earlier on. 

We also appreciate the WP XACT air forks and WP shock on the 125SX. The weight savings from air forks are especially appreciated on the 37-horsepower steed.  

The 125SX handles exceptionally well! The WP suspension is racy and holds up strong. The new chromoly steel frame, with its updated shock, subframe and swingarm, helps lessen squat on acceleration, and it makes the ride height taller around the track. Plus, the fuel-injected engine creates a smoother power delivery that enhances the traction of the rear wheel and the handling as a whole. 

Q: WHAT DID WE HATE?

A: The hate list.

(1) ECU. The locked ECU would be fine if the 125SX ran perfectly, but it doesn’t. It has detonated on us multiple times with pump gas, and whenever we tried a Pro Circuit pipe and silencer, the detonation got worse, even with race gas. But, it can be remap if you find a tuner who has the secret Italian program.

(2) Weight. We are sad to report that the fuel-injected 2024 KTM 125SX gained 10.5 pounds over the carbureted 2022 model. It now weighs 204-1/2 pounds. 

(3) E-start. Nobody on a 125 wants an electric start. It’s easy to kick over a 125.

(4) Big-bore. We thought that KTM would have a 150cc kit ready by 2024, but it is just now being made available from the PowerParts catalog.

(5) Mapping. The bike comes with two maps, but you should avoid the green map at all costs. KTM says that it is a “safety” map, only to use if the bike is detonating, but we’ve had the bike bog randomly on us in this map—not good.

Q: WHAT DID WE LIKE?

A: The like list. 

(1) Engine. The KTM 125SX engine has a smooth roll-on power that makes it easier for anyone to ride, even without two-stroke experience. 

(2) Break-in. It’s nice that the 125SX frame breaks in quicker than the 2024 KTM 450SXF frame.

(3) Suspension. The adjustable-by-hand rebound and compression clickers make it easy to dial in the suspension. 

(4) Brembo. Compared to the BrakTec components on GasGas and Nissin components on the Yamaha, the Brembo brakes and clutch are heavenly. 

Q: WHAT DO WE REALLY THINK?

A: If you’re looking for the best 125 to race Amateur Nationals in the Schoolboy 1 class, or to race 125 money class events like the Pasha 125 Open races, the new-generation KTM 125SX isn’t ready for the big time yet. KTM still needs a little more time to work out the kinks to surpass what they sold in 2022. However, for those who aren’t wannabe Pro racers, this KTM 125SX is very fun to ride and smooth to get into the power on the bottom end.