What’s that? An all-new two-stroke? Yep, the KTM 250 SX is practically all new for 2017 — if you thought two strokes were dead and buried you were wrong
Story Shane Booth | Photography Matt Bernard
I’ll admit straight up I’m a two-stroke fan, partly because I raced them for a lot of years but also due to the enjoyment I get out of riding them. They are a little more brutal — you need to milk the power out of them with more precision than you do a four-stroke and that to me is a fun challenge.
So, when a manufacturer releases a totally new two-stroke machine, that’s exciting news, and I was hanging to throw a leg over the new 250 SX.
WHAT’S NEW?
Well, the list of what isn’t new would be shorter. Here’s a rundown of what is new on this bike. In 2016 every other SX model received a major overhaul but the 250 SX missed out, so for 2017 it gets all of those generic SX range updates, too.
ENGINE The new KTM 250 SX engine features an all-new architecture with a new shaft arrangement for better mass centralisation, a new cylinder with a twin-valve-controlled power valve, a new counterbalance shaft for fewer engine vibrations, a five-speed transmission and a hydraulically operated DDS clutch. The compact 250cm³ powerhouse is distinguished by outstanding and aggressive performance, making it the right choice for everybody who loves low purchasing and maintenance costs as well as the typical two-stroke sound and power.
ENGINE CASES For the new engine generation, the KTM engineers designed an all-new die-cast engine case, raising the clutch shaft by 8mm and the crankshaft by 27mm for enhanced mass centralisation and the corresponding rideability benefits. In addition, it allows the implementation of a counterbalance shaft. The newly designed engine covers feature an advanced surface structure in order to reduce the wear caused by the rider’s boots.
COUNTERBALANCER SHAFT The new engine design allowed the implementation of a lateral counterbalance shaft in order to drastically reduce the vibration levels by a full 50 per cent in the handlebars and footpegs.
CYLINDER The newly developed cylinder of the 250 SX with a bore of 66.4mm has all-new ports and is fitted with a new power valve system. It features a sophisticated mechanism for the lateral support exhaust ports, providing a smooth and controllable response. Two additionally supplied power valve springs with different rates can be switched in within a few seconds to adjust the power delivery to different tracks and rider preferences. Combined with a newly designed cylinder head, the sophisticated exhaust control is essential for the significant performance increase over the entire rev range. The new layout of the water jackets in the cylinder and head guarantee enhanced heat dissipation, which is important when the going gets tough. New lateral cylinder head brackets allow a more efficient fixation of the engine to the frame via new head stays that also reduce the vibration levels.
CRANKSHAFT The new crankshaft provides the same 72mm stroke as its predecessor but has been rebalanced to suit the new counterbalance shaft for minimum overall vibrations.
CARBURETOR The 250 SX now comes with a Mikuni TMX 38mm flat-slide carburetor, improving performance and decreasing sensitivity toward environmental factors like altitude and temperature.
EXHAUST PIPE An all-new exhaust pipe was developed for maximum performance with the new motor.
TRANSMISSION The new engine is coupled to a redesigned five-speed motocross gearbox with nearly the same gear ratios as the previous model. A revised shift mechanism provides more precise and easier shifting while the advanced KTM “No-Dirt” gear lever design prevents dirt from blocking the joint of the lever.
FORK All 2017 full-size SX models are fitted with the super-lightweight WP AER 48 front forks, 48mm upside-down air-sprung forks developed by WP in close collaboration with KTM.
HOUR METER For 2017 an hour meter is neatly fitted to the top triple clamp as standard.
BRAKES & CONTROLS Brembo brakes with lightweight Wave discs are standard. For 2017 the rear brake features a 10mm-longer brake pedal along with new, less aggressive brake pads for enhanced control and overall brake performance. There is also a new throttle assembly with easy free-play adjustment.
BARS & GRIPS Neken taper-style handlebars are fitted to rubber handlebar mounts that have two positions. The grips are ODI lock-on units that require no glue or safety wire.
TRIPLE CLAMPS The CNC-machined upper triple clamp has been redesigned along with the new fork tubes, contributing to the enhanced fork action with reduced friction.
FOOTPEGS The no-dirt footpegs prevent jamming up the pegs, even when digging in deep ruts. If you’re looking for a higher footpeg you can always mount a set of pegs from the 2017 EXC models, which are 6mm higher than the SX versions.
FRAME The frame is an all-new design that’s lighter and provides more torsional rigidity and less longitudinal stiffness.
SUBFRAME The subframe is a more compact design that is also lighter.
SHOCK The shock is now the latest WP offering, bringing this bike in line with the rest of the WP range.
AIRBOX With the updated bodywork and chassis comes the new toolless airbox design.
SWINGARM The swingarm has been redesigned internally so its flex characteristics match up with the new frame.
WAKE THE BEAST
This bike instantly feels light to me as I spend most of my time on 450 four-strokes. As soon as you lift it off the stand it’s noticeable. It’s probably all in my head but it even feels light and nimble just sitting on it. A few kicks and this bike fires up and you can’t miss the fact that there’s very little to no vibration through the handlebars — it’s massively noticeable and in a great way. It makes the two-stroke feel a little more refined than usual.
ON THE TRACK
This engine is a new type of two-stroke. It has a broad power delivery that is still very strong, not watered down to stretch it out. The way it delivers power is the closest to a four-stroke feel I’ve ever felt from a two-stroke.
The bottom and midrange on this bike are very impressive — smooth and a much broader power window than what you might expect. There’s a roll-on feel to the power delivery that means you can get away with less clutch use than you might normally need from a 250 two-stroke.
It’s by no means mellow, either; if you build some rpm and unleash the power with some clutch you better have some traction because it’s still a two-stroke and that will unleash the beast. The major performance gains are in the bottom-to-mid part of the power delivery, which is where you ride the bike the most and where you want controllable power. The top-end performance feels similar to the previous bike.
The new-found torque does allow short shifts though and the engine will pull it most of the time. I found I shifted through to fourth gear pretty quickly on this bike, maybe in an effort to keep the engine in that meaty midrange power, but it may be possible to drop a tooth on the rear sprocket to help the gears last a little longer. There’s no doubt this is, dare I say it, the best stock 250cc two-stroke engine I’ve ever ridden.
DOES IT HANDLE
Benefiting as it does from the chassis updates that the rest of the SX range received in 2016, the short answer is yes, it does handle. It’s also very light on the track.
I actually also had my FC450 out at the track on the day and it was amazing how much lighter the 250 two-stroke rode, especially in the sand. I hadn’t ridden two-stroke versus four-stroke back to back like that for a while and the two-stroke owned the sand turns from a cornering point of view; it stays so much lighter than the 450.
It’s also just so easy not only to put where you want on the track but also to pull back in line if it steps out. It felt almost effortless to wheelie bumps or whoops and then just tip into turns. I ran the WP fork with 149psi and it felt good, holding up well in the braking bumps and sand turns. There was no harshness coming through at all; the rear of the bike matched up nicely.
It’s crazy when you consider how well this bike rides and how nice the engine is yet it doesn’t really have a place in serious competition, which kind of makes it hit home how impressive the 450s are now. There’s no way you can ride a KTM and not mention how nice the controls are: the Brembo brakes feel and power that isn’t found anywhere else on a production brake system. The hydraulic clutch has been refined over the years and makes it tough to go back to cable; the ODI lock-on grips are also a nice touch.
TWO-STROKES ARE HERE TO STAY
After a day on this bike there’s no doubt that two-strokes are here to stay. This is a real-deal developed machine that’s a big improvement on the previous model. It’s been a while since you could say that about a 250 two-stroke motocross bike.
2017 KTM 250 SX SPECS
Engine: Single-cylinder, 2-stroke
Displacement: 249 cm³
Bore/stroke: 66.4×72mm
Starter: Kickstarter
Transmission: 5 gears
Fuel system: Mikuni TMX 38
Final drive: 14:50
Cooling: Liquid cooling
Clutch: Wet multi-disc DDS-clutch, Brembo hydraulics
Ignition: Kokusan
Frame: Central double-cradle-type 25CrMo4 steel
Subframe: Aluminium
Handlebars: Neken, Aluminum Ø 28/22mm
Front suspension: WP-USD, AER 48, Ø 48mm
Rear suspension: WP shock absorber with linkage
Front/rear brakes: Disc brake 260/220mm
Front/rear rims: 1.60 × 21″; 2.15 × 19″ Excel
Front/rear tyres: 80/100-21″; 110/90-19″
Steering head angle: 63.9°
Triple clamp offset: 22mm
Wheelbase: 1,485 ± 10mm
Ground clearance: 375mm
Seat height: 960mm
Fuel capacity: 7L
Weight, ready to race: 95.4kg