We got the distinct feeling that KTM never wanted to make a 300SX two-stroke. It’s as if the guys on the off-road side of the company were protecting their turf. The 300 two-stroke was an off-road bike; motocross guys can have everything else! Way back in 2002 there was a 300cc motocross bike that coexisted with a 380SX, but for the next 21 years, the biggest two-stroke that KTM offered the motocross world in the U.S. was the 250SX. During that time, KTM offered a 300 kit, but it didn’t really outperform the 250SX unless you combined the 300 top end with the 250 black box. That increased the revs dramatically, but gave the bike a notoriously short fuse. Then came the 300SX.
When the 2023 KTM 300SX arrived, it was the biggest two-stroke motocross bike imported to the U.S. Now, it’s been joined by a Husqvarna TC300 and, if you know where to look, a TM MX300, but there is still no GasGas. KTM is still testing the waters, but the marketing guys figured that the time was right, and the engineers felt the technology offered by the new throttle-body two-stroke platform would lend itself well to motocross. This generation of the 300SX represents a radical departure from traditional two-stroke thinking. The bike’s CPU controls everything from the power valve opening to spark advance to fuel delivery. It’s all mixed together in a complex multidimensional map. As of right now, KTM doesn’t like the end user getting into that map and altering it. There’s a handlebar-mounted map switch that offers a mild setting (white light on) and an aggressive setting (green light on), but the options end there. There is no traction control or Quickshift like the four-strokes, and there’s certainly no access to the mapping through the Ride KTM smartphone app. As is, there’s a substantial difference between the two maps. With the white light on, the power-valve throw is limited to 80 percent of its full travel. In green mode, it’s allowed to fully open. The mapping appears to be a work in progress. There were updates immediately after the bike was introduced, and the 2025 version, apparently, is revised as well.
The company is still very conservative when it comes to engine life. The service manual asks that you replace the piston every 15 hours—not inspect, replace. The piston replacement interval for the 300XC-W trail bike, on the other hand, is 40 hours. The two bikes employ the same technology, but the motocross bike is in a much higher state of tune. That’s not just from mapping, either. The 300SX has a different cylinder and head with different port timing. As for the chassis, the 300SX is pretty much the same as the four-stroke KTM models. They all have an updated frame for 2025 with a little more flex built in. This is a reaction to all the criticism that came in 2023 when everything was changed. The chassis of that period was influenced by the Red Bull KTM Factory Supercross team and was very stiff. The 2025 frame has less gusseting and more flex. Not surprisingly, some of the factory Supercross riders still prefer the stiff frame from last year. Tom Vialle quietly continues to use the older frame on his 250SX-F, at least in Supercross.
Up front, a WP Xact air fork is used, and it received subtle changes for 2025 to work better with the new frame. Same with the WP shock. With these slight changes, the bike did lose a touch of weight. On our scale, it’s 221 pounds without fuel.
TWO-STROKE CHAMPIONSHIP TIME
We can’t be trusted to save a test bike in new condition for very long. Bikes arrive, they get ridden, and they don’t often get the best of care. The 2025 300SX was different. It sat in the Dirt Bike shop for months with a big “DO NOT RIDE” sign hanging on it. It was being saved so Mike Brown would have something to ride in the Wiseco Two-Stroke Championship at Glen Helen. As it turned out, he couldn’t come this year. There was a mad dash to ride the 300. We thought we knew the bike. Our test of the 2023 version was hot and cold, and we expected the 2025 model to be the same. It turns out that the subtle changes that have taken place made a very big difference.
First things first, this is probably the one bike in the KTM line that benefited most from the new frame. With the social-media-driven frenzy of hyperbolic opinions, you might think the previous frame was positively medieval. In truth, it wasn’t that bad once it was broken in, at least on the four-strokes. It didn’t, however, jell well with a two-stroke power delivery. At the risk of offending the faithful, two-stroke motocross bikes can be a handful. The sharp power delivery has ripple effects in handling; they tend to be harsh and magnify any negative traits. The rigid frame of 2023 wasn’t a good way to introduce the first 300cc motocross bike of the decade.
The 2025 300SX has a much kinder disposition. It’s not just about absorbing bumps; the new bike has more stability and provides more confidence. You don’t get as much arm pump, and you can let the suspension do its job. As far as suspension performance goes, there’s no reinvention of the wheel here. The WP air fork still does a decent job, but doesn’t generate any rave reviews. It still ranks as the best production air fork of the modern era. It is, of course, the only remaining air fork after the hype of 10 years ago subsided. WP was clearly the victor in that contest and has made subtle improvements since then. Nowadays, however, there’s an increasing gap compared to the best of the coil-spring forks. The KTM fork is a little harsh on sharp edges and doesn’t provide the most level platform as you enter, accelerate, and exit any given turn. And, no one is particularly fond of checking air pressure before every moto. The rear suspension of the KTM rates much higher in most opinions. It holds no surprises and provides consistent performance throughout a long moto.
RING-DING VERSUS THUMP-THUMP
In our last chapter, we declared the 300SX a beast. In its aggressive map, the 2023 model hit hard, signed off early and left riders a little frazzled. We didn’t expect the new bike to be much different. But, it is. Evidently, the mapping changes that have come since then have resulted in a longer, usable powerband. The meat of the power starts earlier and lasts longer. Initially, we thought the bike might have been slightly detuned since its roll-out. That’s not the case. The 300SX still makes peak horsepower in the low 50s. That encroaches on 450 turf. Now, that peak isn’t quite as brief, so you can carry each gear a little longer.
Having said that, we have to offer a little perspective. If you’re a rider who’s grown accustomed to four-strokes, the 300SX might be a shock. A powerful two-stroke, even one with a relatively smooth delivery like this, is a bit of a toggle switch compared to a modern four-stroke. You have to be in the right gear at the right time to get the most out of it. Jumping, in particular, requires careful throttle control and planning. If you’re sloppy, you can have 50 horsepower arrive on the face of the jump and send you into the next county. And, on acceleration, you can’t just pound through the rough stuff. For reasons that Issac Newton could probably explain, the increased spinning mass within a four-stroke motor provides a little more straight-line stability. Conversely, two-strokes are much easier to toss around and bend to your will.
THE BIG PICTURE
Presumably, two-stroke lovers know all that stuff. That’s part of the attraction and why the faithful remain faithful. The 2025 version of the KTM 300SX isn’t necessarily a game changer. The big picture is still the same, and two-stroke motocross remains a subset of a subset. But, KTM is still devoted to the idea that two-strokes should be a part of that picture. Racing a 300SX requires more than passive involvement. You have to learn how to ride a little differently and commit to a more demanding maintenance schedule. In exchange, it takes the concept of two-stroke motocross to the highest level it’s ever attained.