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Dirt Bike Magazine - Best selling off road motorcycle magazine

This year KTM tossed everything about its existing two-stroke motocross bike in the trash. The motor in the previous SX two-stroke had served KTM well. It had received a major update in 2017, but hadn’t offered technology or concepts that were truly new. It still had a Mikuni carburetor, a kickstarter and a ball-ramp mechanism working a mechanical power valve. All those things had been around since the ’70s. Now, KTM has started over.

One of the most superficial changes is the new displacement. There was no 300cc motocross two-stroke in KTM’s American line-up last year. There were 300cc off-road bikes and a 300cc kit for the 250SX motocross bike, but the motor wasn’t truly designed to be that displacement. Riders who installed the 300 kit on the 250 ran into problems with the ignition and a lack of rpm. If they got around those issues, they had reliability issues. KTM was also reluctant to bring fuel injection to a two-stroke motocross bike. Their engineers had invested a great deal of time and development into Transfer Port Injection (TPI), but that always seems more appropriate for an off-road bike. KTM didn’t use electric start or an electronic power valve, either. They said they liked the simplicity of the existing bike, which incorporated a ball-ramp power valve and a kickstarter.

The 2023 motor was designed around throttle-body fuel injection, which is basically what a four-stroke uses. Transfer Port Injection and oil injection remain only on the off-road bikes. The electronics package was further complicated by bringing electric start and an electronic power valve. Electronic power valves have been used before, but we haven’t seen anything like this. Previous designs have acted like mechanical power valves. They were open or shut. All that engineers could work with was how quickly they opened and at what rpm. KTM’s new system is driven by the same CPU that operates the fuel mapping and spark advance. It looks at rpm and throttle position to determine when and how far the power valve should open. That means the final mapping is far more complex than anything seen previously on either the two-stroke or four-stroke.

All of KTM’s 2023 full-size motocross bikes got a new chassis, including this one. The frame is more rigid and has a new shock and rear suspension layout. The front suspension is still a WP Xact air fork, and the hydraulics are still by Brembo, including brakes and clutch. When it was all done, the new bike did gain some weight. The previous 250SX was 215 pounds. Now, with the battery, the starter and the new frame, the KTM 300SX is 223 pounds.

RETURN OF THE OPEN-CLASS TWO-STROKE

We love to toast the sheer power of old-timey 500 two-strokes, but in reality many modern 450 four-strokes have attained horsepower numbers in the same vicinity—somewhere in the mid 50s. The 500 two-stroke got there with torque and modern 450 four-strokes do it with rpm. The 300SX does it with both. At low rpm, the 300SX has incredible pulling power. It’s almost startling, but that’s nothing compared to what’s waiting later. In the mid-range, the 300 goes a little crazy. It hits with strength and venom, so you better be prepared. The motor pulls so hard and goes through the middle rpm level so fast that most riders chicken out and upshift early. If you try to roll off just a little, you end up in a very awkward place where it’s actually difficult to dial in the right amount of horsepower. If, on the other hand, you stay in it all the way to peak revs, you end up accelerating as hard as any motorcycle made. The 300 still doesn’t produce the peak power or rpm of a 450 four-stroke, but it’s close enough. You just have to have someplace to go, and most motocross tracks aren’t made of straight lines.

If that sounds a little intimidating, that’s okay. We like motorcycles that scare us a little. The trick with the 300SX is learning to use that crazy zone in the middle. For many parts of the track, you can upshift and let the torque do all the work for you. If you enter a corner at low-to-middle rpm, you can pull most of the way through without touching the clutch. Then, once you’re strapped in and ready to go, you hit the clutch once or just wait for the fireworks to commence. Bring your A-game. It’s an absolute blast when you do it right. But, as we all know, a few laps into a 20-minute moto your A-game starts slipping away. The engineers and test riders at KTM know that, which is why the 300SX has the sissy switch. Other riders might call it a map switch. There are two buttons—one activates a green map and the other is a white map. Ride with the green map when you’re feeling strong and wanna have fun, then go to the white map when you’ve had enough. The white map tones down the hit considerably. It also shaves a little power off the top. It’s the smart map to use most of the time, but be forewarned: once you’ve tasted the wild side, it’s hard to go back.

KTM went all in with a new frame for 2023. Between the KTM and Husqvarna lines, the frame comes on over 20 new models. That frame doesn’t make friends easily. Initially it comes across as stiff and unforgiving. After a while, it grows on you. One issue is that it “breaks in” and gets more compliant with time. Another is that its greatest assets are more apparent in extremely rough conditions that don’t show up that often, whereas its stiffness is apparent on the ride to the start line.

The frame for the 300SX isn’t exactly like the one used on the four-strokes, but it’s very, very close. It has the same steering geometry as the old one, which means it is excellent in turns. It drops in easily and allows you to make adjustments mid-turn. But, it gives you the same initial feedback as the four-stroke frame—it’s very stiff on small impacts and simply not as comfortable as the older, flexier frame. Additionally, there’s the two-stroke factor in choppy terrain. You don’t get the invisible hand of gyroscopic stability that four-strokes offer. On acceleration, sharp edges make the bike deflect more. The hard-hitting power delivery combines with that to make traction difficult to find and keep.

What that means to a racer is that there are times when he’s going to have to work a little harder than his buddies on 450 four-strokes. The rougher the track, the more the two-stroke will struggle to put all that power to the ground. On the flip side, a track with good traction and fewer square-edge bumps is 300SX heaven. When you’re all lined up with smooth chocolate dirt before you, there’s nothing as satisfying as feeling that green-map power surge take you to the next turn in warp drive.

So, even though the new 300SX changes the two-stroke game in motocross, it remains a very specialized bike. It offers sensations and feelings that no four-stroke can duplicate. On the right track, it can take on a modern 450 head to head. Once you start considering the real world with a cross-section of flawed, ill-prepared tracks, however, it’s clear that the 300SX is a bike that will end up being more fun to ride than it is to race.

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