TEST UPDATE: 2023 KTM 125SX TWO-STROKE — A FIRST-YEAR MODEL WITH QUIBBLES

On May 11, 2023 Last updated May 11, 2023

HOW’S THE ’23 KTM 125SX ENGINE?

The 2023 KTM 125SX engine is all new, and this is the first time that KTM has fuel-injected its two-strokes with a throttle body injection system (rather than transfer port injection). It’s also the first time they’ve fuel injected their motocross bikes. Before, only the off-road two-strokes had TPI fuel injection. The fuel-injection aspect makes it easier to roll into the power, whereas with the carbureted ’22 model, it took more effort to find the power’s sweet spot.

HOW FAST IS THE ’23 KTM ENGINE ON THE DYNO?

It’s not as fast as last year’s 125SX, which was identical to the 2023 GasGas MC125. The new engine is slightly slower off the crack of the throttle. At 7500 rpm, the KTM, Husky, and GasGas 125s are exactly the same until 10,800 rpm, where the GasGas continues to climb and the fuel-injected KTM/Husky 125s start to taper off. The KTM peaks at 37.21 horsepower at 11,200 rpm, while the GasGas peaks at 38.25 at 11,400 rpm.

COULD THE ’23 ENGINE BE BETTER?

Yes. The biggest complaint is with the locked ECU box. We expected to see Vortex come out with an aftermarket ECU for the 2023 Austrian two-strokes, but, because the Austrian two-stroke ECUs come from a different manufacturer and use completely different programs from the four-stroke KTM/Husky/GasGas models, Vortex doesn’t have the hardware (or software) to make a replacement ECU. It’s a big investment for Vortex to make a new setup for these bikes, and so far, they are holding off.

Because the stock ECUs are locked, there’s no way for anyone to remap these bikes, which is detrimental for aftermarket tuners. Typically, tuners adjust jetting in carburetors or play with mapping on fuel-injected bikes to work with their engine mods, but the locked ECUs are making this difficult. WMR Motorsports in Florida just cracked the code on these ECUs, and so far they are the only shop able to adjust the mapping.

The ’23 KTM 125SX engine could be better if tuners had access to the stock mapping. Our 125SX has had trouble with detonation on multiple occasions.

HOW GOOD ARE THE 125SX FORKS?

Very good! The WP XACT air forks are plush and much softer than 250SXF forks but stiffer than Husqvarna TC125 and GasGas MC125 forks. Out of the box, the 125SX forks were great for a wide range of riders.

WHAT ABOUT THE REAR SUSPENSION?

The WP shock is awesome. It’s easy to set up, and now it has adjustable-by-hand high- and low-speed compression adjusters; however, the rebound adjuster is awkward to reach, and we recommend using a flat-head screwdriver to adjust it still.

HOW DOES IT HANDLE?

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 all the way 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.

WHAT DID WE HATE?

(1) 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. (2) We are sad to report that the KTM 125SX gained 10.5 pounds. (3) The 125SX doesn’t need an electric start. (4) 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.

WHAT DID WE LIKE?

(1) When it’s not detonating, the KTM 125SX engine has a smooth, roll-on power that makes it easier for Novices to ride—the smoother power makes this bike more accessible. (2) We’ve complained a lot about the break-in time required for the 2023 KTM 450SXF. Thankfully, the break-in period was short and sweet on the 125SX.

WHAT DO WE REALLY THINK?

If you’re looking for the best 125 to race, the KTM 125SX isn’t it. KTM needs a little more time to work out the kinks, and the aftermarket companies need more time to learn about this bike. If you aren’t as worried about racing and you don’t mind leaving the bike stock, you can have a lot of fun on this 125.