TWO-STROKE TEST: 2012 KTM 150SX IS ALL ABOUT HAVING FUN ON A DIRT BIKE

A: No, the KTM 125SX motocross bike is back for 2012. Three years ago, KTM made the 125SX obsolete by introducing an identical motocross bike with more power?the 144SX. After a quick glance at sales figures for the 144SX versus the 125SX, KTM decided to shelve the 125SX and only import the 144SX, which would later morph into the 150SX. For 2012, KTM decided that the market for a true 125cc two-stroke was big enough to put it back into the lineup.

Mechanical parts: The 150SX powerplant produces a healthy blast of power, albeit across a much narrower range than a 250cc four-stroke. Gun-and-run is the best strategy.

Whoa: KTM’s 260mm front brake is powerful on a 243-pound 450SXF, so you can imagine how well it works on a 150SX.

Jetting: We go richer on the mainjet and always run premium gasoline. At even the slightest ping we raise the needle.

Q: WHAT CHANGES DID KTM MAKE TO THE 150SX FOR 2012?

A: The addition of rising-rate linkage is the biggest individual change for 2012. The linkage also acted as a catalyst for other changes. There were 11 changes in all (five of them related to the change from no-link suspension to rising-rate linkage).

(1) Linkage. Gone is last year’s PDS suspension system, which ironically had been totally redesigned for the 2011 model year. In its place is the rising-rate linkage off of the KTM four-strokes.

(2) Shock. The new two-stroke WP shock is 73mm longer and has a 28mm-longer stroke than the PDS version. It is the exact same shock as on the KTM 250/350/450SXFs, except with lighter damping and spring rates.

(3) Frame. In order to accommodate the new shock, the frame needed a new, repositioned upper shock tower and a bracket on the bottom for the linkage to bolt to. Last year, KTM made geometry changes, including a steeper head angle, and those changes remain.

(4) Swingarm. Obviously, the swingarm had to be altered to accept the central mounting of the rising-rate linkage. The swingarm is a one-piece cast unit that has new flex characteristics, along with mounts for the linkage.

(5) Air boot. The old PDS shock was mounted off to the right side of the centerline, which opened up room on the left side of the bike for a straight path to the carb for the air boot and airbox. That advantage has been lost with the linkage system, which requires the shock to be centered in the frame. KTM had to design a new air boot to fit in the tighter confines. The new air boot has sweeping, rounded curves, whereas the old unit had sharper angles.

(6) Cylinder head stay. There is a new cylinder head stay that is designed to decrease vibration.

(7) Suspension. The WP forks and shock get new seals, bushings and damping settings.

(8) Kickstarter. The kickstarter’s casting has been reshaped for more strength. This is a carryover improvement that was really designed for the 300XC, but was transferred to the 150SX.

(9) Pipe. The exhaust pipe has been tuned for broader power so that it gets through the midrange and into the racing part of the powerband quicker. It is not the same as the 125SX pipe.

(10) Tires. The old Bridgestone M59/M70 tire combo has been replaced with Pirelli Scorpion Mid Soft tires.

(11) Bodywork. The airbox and rear number plates are now white. The rear fender has been reinforced to decrease flex. Finally, there is a new Renthal crossbar pad.

Q: HOW FAST IS THE KTM 150SX ON THE TRACK?

A: It’s quick. With a little practice carrying speed exiting turns, a KTM 150SX rider can turn competitive lap times and get more giggles per straight than on any other bike. When the 150SX is hooked up and churning, it can pull away from a 250F. It is getting it hooked up on hard dirt, off-camber and parabolic turns that is the hard part.

With its narrower powerband and greater tendency to spin the rear tire, the two-stroke made MXA test riders more prone to mistakes than the four-stroke. The four-stroke’s consistent, albeit rather pedestrian, performance upstaged the two-stroke under certain circumstances. It was easy to spin fast laps on the KTM 150SX, but it was also easy to fall off the pipe in tricky situations, resulting in slower lap times.

Compared to a 250 four-stroke, the KTM 150SX has a narrow powerband that requires an all-out-attack strategy and a finger on the clutch at all times. The quick bursts of power available from the 150SX engine were great on sand, loam, whoops and rough ground. No matter what, a KTM 150SX rider always had the giggle factor generated by the extra sensation of speed.

Q: HOW FAST IS THE 150SX ON THE DYNO?

A: Our 2012 150SX peaked at 38.83 horsepower at 11,100 rpm. For comparison, our 2011 KTM 150SX made a little over 40 horsepower. If we were betting men, we’d gamble that the 2012 air boot is where KTM lost the power.

People may say that 250 four-strokes make more power than 150 two-strokes, but it’s not true. The only 250F that we have tested that makes more than 38.83 horsepower is the 2012 KX250F. All the rest make less power than the KTM 150SX (as much as three horsepower less).

How does the KTM 150SX compare to the Yamaha YZ125? It blows it away. Peak horsepower on the 150SX is 38.83, while peak on the YZ125 is 34.01 horsepower.

Q: IS THE 2012 KTM 150SX COMPETITIVE WITH THE 2012 KTM 250SXF?

A: Yes, very competitive. The 150SX makes 3.35 more horsepower at peak than the KTM 250SXF. Everywhere below 10,000 rpm, the thumper makes a lot more power, but in its sweet spot, the 150SX pulls away. One thing that helps the 150SX go faster is that it weighs 25 pounds less than the 2012 KTM 250SXF. You don’t have to wrestle with the 150SX. It is a joy to ride across rough ground, because it feels like it floats.

In MXA‘s opinion, the KTM 150SX has made significant steps forward, while the KTM 250SXF has taken a couple steps backward. Past model years have proven that KTM can make an incredibly fast 250F engine, but this year they lost horsepower. Plus, even though the KTM 150SX two-stroke gained weight with the addition of rising-rate suspension, the KTM 250SXF gained three times as much weight with fuel injection (5 pounds), rising-rate linkage (5 pounds) and electric starting (5 pounds).

Q: HOW WAS THE GEARING?

A: In years past, we have geared the 150SX down one tooth on the rear. This helped the KTM 150SX get on the pipe quicker, get to third gear sooner and stay in the sweet spot of the powerband with less effort. We still think the 51-tooth sprocket (stock is 50) is the best overall option, but we did occasionally find ourselves between gears with this setup. Some test riders wanted to go to a 52-tooth sprocket. Beware, the 52-tooth sprocket only works if you are committed to getting into third gear quickly. If you try to ride it in second gear, you will corkscrew into the ground.

Q: WHAT WERE OUR BEST JETTING SPECS?

A: We have always felt that the KTM 150SX is on the lean side?and it only gets worse in cool weather. Here’s what we ran (stock settings in parentheses).
   Mainjet: 185 (182)
   Pilot: 40
   Needle: NOZI
   Clip: 2nd from the top
   Air screw: 1-1/2 turns out
   Notes: In cool conditions, we would definitely consider a 190 mainjet and a 42 pilot.

Q: IS THE LINKAGE REAR SUSPENSION BETTER THAN PDS?

A: Yes and no. It depends on your priorities. Most of the time, the action of the linkage rear suspension feels very similar to the old PDS system. When testers first threw a leg over the linkage-equipped KTM, they didn’t immediately notice a big difference. Yet, over time, it became obvious that the linkage system was more consistent at high shock-shaft speeds, didn’t spike at the extremes of travel, was easier to set up and less sensitive to spring rate.

Q: WHAT ARE MXA’S RECOMMENDED SHOCK SETTINGS?

A: Here is what we ran (stock settings are in parentheses):
   Spring rate: 4.7 kg/mm
   Race sag: 100mm
   Hi-compression: 2 turns out
   Lo-compression: 15 clicks out
   Rebound: 15 clicks out
   Notes: Whereas the performance of the previous PDS system was entirely dependent on having the correct shock spring (which was never the one the bike came with), the new system fits a wider range of riders. If you have less than 35mm of free sag, consider switching to the next stiffest spring.

Q: HOW GOOD WERE THE WP FORKS?

A: We didn’t notice much difference with the new seals, bushings and settings. The slightly soft spring rate masked the WP’s midstroke harshness and let our lighter testers get by with simpler clicker changes. For faster or heavier riders, a better setup would be to go firmer on the fork springs and fiddle with the oil height to get the midstroke damping that suits the situation. One of the great things about a light, agile and snappy two-stroke is that it places fewer demands on the front end, so MXA test riders were more tolerant of flaws. The forks were workable with just clicker changes.

2012 KTM 150SXF: KTM deserves credit for bucking the system by building bikes with displacements outside the normal 125cc, 250cc and 450cc regulations. Bikes like the KTM?150SX add spice to the market place.  

Q: WHAT ARE MXA’S RECOMENDED FORK SETTINGS?

A: Here is what we ran in our 2012 KTM 150SX (stock settings are in parentheses):
   Spring rate: 0.42 kg/mm
   Oil height: 350cc
   Compression: 6 clicks out (12 clicks out)
   Rebound: 10 clicks out (12 clicks out)
   Notes: These forks are very soft, but given the target weight of the typical 150cc two-stroke rider, we think this is the proper choice. Faster or heavier riders will need 0.44 fork springs.

Q: HOW DID THE 150SX HANDLE?

KTM’s WP front forks are set up soft on the 150SX because the bike is very light, compared to a 250 four-stroke, and the target riders are not behemoths.

Q: HOW WERE THE TIRES?

A: All things considered, the Pirellis are quality tires that perform well. Compared to the old Bridgestone M59/M70 combo, the Pirelli Mid Softs favor a softer, looser type of dirt. They seem to last a bit longer than the Bridgestones did?unless they are ridden on rock-hard dirt, in which case the front tire sheds side knobs. They weigh about the same as the old Bridgestones and are lighter than the Dunlop MX51s spec’ed on the 2012 KTM four-strokes.

Q: WHAT DID WE HATE?

(1) Weight. Yeah, yeah, yeah, we did rave about how light it is, but that is because it is anywhere from 18 to 25 pounds lighter than the four-stroke squadron. Sadly, the KTM 150SX is 5 pounds heavier than it was one year ago. Both the 2012 Yamaha YZ125 and TM MX144 are lighter than the KTM 150SX.

(2) Fender cleaning. The white rear fender looks good, but can show mud residue after cleaning if not hand-scrubbed. The extra bracing material is functional, but it can trap a little dirt.

(3) Preload ring. Even gentle prying deforms KTM’s plastic preload ring. The clamp system is handier than the lock-ring system, but the nylon needs to go.

(4) Gas cap. We keep complaining about the gas cap, because we know that KTM is reading this. Putting a lock on a gas cap, especially one that requires two hands to use, is like wearing suspenders to hold your belt up. Come on, KTM, make a true-to-life gas cap and put a lip on the gas tank so that dirt doesn’t fall down into the fuel.

Nothing flies like a lightweight two-stroke. You have to loft it into the air, but once it achieves optimum trajectory you can do with it as you please.

Q: WHAT DID WE LIKE?

A: The like list:

(1) Front brake. This little bike doesn’t need all the power that a 260mm Braking rotor and Brembo caliper have to offer, but it sure is fun to squeeze.

(2) Clutch. The precise control of a cable-actuated clutch can be an advantage when nursing a 250cc or 300cc two-stroke into the desired part of the powerband, but with a 125/150, the best method is grab-and-go. The hydraulic Magura clutch is smooth and consistent throughout the longest motos.

(3) Silencer. We love the nylon hanger bracket that KTM uses to hold the silencer to the frame. Super cool. Sadly, we tend to think that aftermarket pipes and mufflers make the KTM 150SX even faster.

(4) Psychology. In a four-stoke-dominated society, the angry buzz of a two-stroke behind him can throw the concentration of a fellow competitor off.

Q: WHAT DO WE REALLY THINK?

A: We’d love to love this bike?which is why we readily admit that we do love it. It’s better than a 125 two-stroke, and in the right hands, it can beat 250 four-strokes. It is, however, more difficult to ride, because it has a narrow powerband and needs to be wrung out. If you ride motorcycles because they are fun, nothing is as fun as a flat-out assault on a motocross track on the KTM two-stroke.