HOW TO BUILD THE ULTIMATE KTM 250SX TWO-STROKE: YES, IT DOES HAVE WORKS SHOWA SUSPENSION ON IT

May 6, 2013
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By Billy Musgrave

I must admit that I inherited MXA‘s KTM 250SX project bike by default. I was third in line, but eventually the end user. Jody had started the project originally, but he had other assignments and handed it over to my father, Willy Musgrave. I took over the project when my dad crashed at Glen Helen and broke his collarbone. It might seem mercenary, but it was a lucky break for me.

I have been racing MXA‘s 125 and 150 two-strokes for many years. Occasionally, I would get assigned to a 250 two-stroke or even a four-stroke, but my personal preference was to race a 125/150 two-stroke against 250cc four-strokes. Being a full-time two-stroke guy has its perks at MXA because they assign you every two-stroke that comes into the workshop. They have specialist for lots of things…I was the two-stroke one. It was fun racing tiddlers, but as a 250 Pro, it really wasn’t a smart decision to specialize in 125 and 150 two-strokes. But like most MXA test riders, I do what I am told and race the bikes they assign me.

I’m not complaining. I loved the KTM 150SX that MXA had assigned me to race and to use as a basis for comparisons against the Yamaha YZ125/144, TM 125/144 and Husqvarna CR125/144. But, getting called up to the “Big Show” after my dad’s crash (he is completely healed now) was a major step up in my racing career. Suddenly, I wasn’t in the middle of the pack on the starts ? with a 250cc two-stroke instead of a 150, I was at the front. The confidence boost of knowing that for the first time in the 250 Pro class I would be the guy with the most horsepower?instead of the guy with the least?was liberating.


The gold anodized hubs come from TCR. In a cool move, TCR?designed them to accept the stock KTM components.

I got off easy when it came to developing MXA‘s KTM 250SX, because by the time I was thrust into the R&D role, the two test riders before me had done most of the work. And although I blanched at the cost of what they did, I love being the recipient of the fruits of their labor.  I don’t think that many people can afford to follow MXA‘s blueprint to the ultimate KTM 250SX, but these are the steps that were taken to improve MXA‘s KTM 250SX. You don’t have to do all of them. You don’t have to do any of them. And if you don’t like buying aftermarket carbs, suspension or shock linkages…make your own in your home mill, foundry and machine shop. We have both a 2013 KTM 250SX and a 2012 250SX?we did the same mods to both bikes (the photos are of the 2012 model..but the front fender is the only noticeable difference between our two bikes)

Step one: I’m a small guy, but even I couldn’t race the KTM 250SX with the stock suspension components. It was undersprung and under-damped in the midstroke. During the initial test process, when every MXA test rider races the bike, I liked the power and handling, but it bottomed hard everywhere.

Jody handled the first steps in working with KTM’s techs on the stock suspension. He got rid of the stock 0.46 fork springs (0.44s on a  2012 model) and 5.4 kg/m shock springs. Upping the springs to 0.48 on the front was a big help (and the bigger test riders went to a 5.7 on the shock), but the damping was still an issue. When Jody had to move to another test bike project, he handed the 250SX over to Willy Musgrave to finish. Willy asked Jody if he could revalve the stock forks and shock, Jody said, “No.” My dad has been a test rider for a long time, and he understands that MXA likes to keep their suspension stock as a control unit for comparison against modified suspension. But Jody eventually took pity on my dad and said, “I have a set of Showa A-kit works suspension made especially for KTMs. It is set up for a KTM 450SXF right now, but most of the settings would be ballpark for your speed and weight on a 250 two-stroke. You can take those if you want.”


The longer Pro Circuit shock linkage stiffens up the initial part of the stroke to hold the rear higher under a load. Additionally, it drops the rear of the very tall KTM 250SX to balance out the fore/aft weight bias. This is a good mod.

What a difference! Actually, it is close to an $8000 difference. Willy had the works triple clamps, forks and shock on the bike in 10 minutes. Tools were flying; I’ve never seen any mechanic work so fast. The setup was light years better than the stock WP stuff (no surprise), and Pro Circuit’s Bones Bacon agreed to take care of any fine-tuning necessary (and we had the components rebuilt and revalved several times)The kicker is that the Showa A-kit suspension (forks, shock and triple clamps) costs $8000. We don’t expect you to pop for so much money, but when you go for the “ultimate” sometimes you have to go big. Please note, that you could revalve and respring the stock WP components 10 times for that amount of money. And, eventually you would find perfection with the stock stuff. But?and this is a big but?they would probably never be as good as the Showa stuff.

Here is what we ran in our stock WP forks…just so you know
    Spring rate: 0.48 kg/mm (0.46 stock)
    Oil height: 370cc (380cc)
    Compression: 7 clicks out (12 stock)
    Rebound: 12 clicks out
    Fork leg height: 5mm up (flush on a sandy track)
    Notes: The cheapest way to fine tune the KTM’s WP forks is to raise and lower the fork oil height to get the smoothest midstroke action without bottoming (which is why we lowered the oil height by 10cc to make them better suited to the stiffer fork springs). Novices and Vets might like the stock fork springs (and might even want to lower the oil height 10cc also, but Pros will need either 20cc more oil with the stock springs or stiffer springs and 10cc less oil). KTM’s forks come with different oil heights for each model. The 125SX and 150SX have their oil set at 360cc, the 250SX has 380cc and the four-strokes roll out of the factory with 390cc.

As for the stock WP shock, we went in this direction:
    Spring rate: 5.4 kg/mm
    Race sag: 105mm
    Hi-compression: 2 turns out
    Lo-compression: 15 clicks
    Rebound: 12 clicks
    Notes: We ran a 1.25mm longer Pro Circuit shock linkage. Not only did it lower the rear of the bike by 10mm, but it stiffened up the initial part of the shock’s stroke to help widen the range of the stock 5.4 shock spring. Faster or heavier riders might be required to go to the stiffer 5.7 kg/mm shock spring. If you are savvy, you will use free sag to make the decision for you.

Step two: In stock trim, the KTM 250SX makes the most horsepower of any 250cc two-stroke sold. It peaks just shy of an amazing 50 horsepower at 8400 rpm. But even with 49.77 horsepower, the 2012 KTM does have a weak link. The power doesn’t do much after peak, and, in fact, drops off to 44 horsepower by 9000 rpm.


The 250SX engine was left stock. It wasn’t ported, milled or big-bored. The complete list of engine mods was limited to the Pro Circuit exhaust pipe, Dick’s Racing Intelajet carb mod, 450SXF-FE Belleville clutch washer and a Kreft Power power valve adjuster.

The simple solution was an aftermarket exhaust pipe. Bolting on a Pro Circuit KTM 250SX pipe and silencer didn’t add any power from idle to 8000 rpm, but by 8500 rpm, it made a half horse more. At 9000 rpm, the Pro Circuit pipe made 5-1/4 horsepower more; at 9500 rpm, it makes an amazing 7 horses more. Don’t read that to mean that the pipe made the 250SX produce 57 horsepower?what it did was produce more horsepower after peak in a place where the power was weak before. Those extra horses on the high end of the rpm scale are the meat on the bones of the 2012 powerband. At $239.50 for the pipe and $119.95 for the silencer, this was a well-spent $360.

 
You don’t see works Showa forks on a KTM every day, but if you have extra cash this is where you should spend it (right after buying food).

 
The VForce3 comes stock on all KTM two-strokes. The Dick’s Racing carb mod was worth its weight in gold.

 
The Pro Circuit triple clamps are required to accept the 49mm Showa works forks. The offset is the same as stock.


Step three:
Jody’s last contribution to this KTM 250SX project before moving on was to call Dick Wilk at Dick’s Racing to have the smallish, 36mm Keihin PWK taper-bored from 36mm to 39mm. Then, Dick added “Intelajet” to the carb and his Quad Flow winglet in the carb venturi.

I had never heard of Intelajet, but when the carb came from Dick’s Racing, Jody, Willy and I went to Glen Helen to test it back to back against a stock 36mm PWK. First, we did laps with the stocker, and then we bolted the Intelajet carb on and went back out. I was flabbergasted. I had never felt such a rush on the top end as with this miracle carburetor. It was like a supercharger. Once the rpm reached two-thirds throttle, the auxiliary fuel flow of the Intelajet’s upstream emulsion tube would kick in. My dad and I were babbling like little kids who had just tasted ice cream for the first time. The gain in top-end power and overrev was immediately noticeable. It had no effect on power below two-thirds throttle, but once the mist got enough vacuum pressure to start working, the KTM 250SX felt like it had a jet pack attached to it. The only jetting change was to replace the stock 160 mainjet with a smaller 152, because the Intelajet emulsion tube spray makes up for any loss in high-rpm fuel. On the dyno the effects of the Intelajet mod was barely noticeable, but on the track it was impressive.

The Dick’s Racing carb mod was not cheap. It cost $425 to taper-bore the carb and install the Intelajet fittings in the float bowl and velocity stack, plus another $150 for Quad Flow winglet work.

Step four:
The KTM 250SX is a European-style 250. This means that it is oddly configured in terms of seat height, spring rates and handlebar bend. It is a rather large bike for a two-stroke. On our 2012 KTM 250SX we ran 5mm taller bar mounts ($27) out of the PowerParts catalog, but in 2013 KTM put taller bars on the bike stock.

The seat height was a problem that we addressed early on in the project. First, when MXA had the Showa A-kit built for the KTM 450SXF, we had Bones Bacon shorten the Showa shock by 4mm (this brings it down to the WP 2013 spec). Then, we added a longer Pro Circuit shock linkage. It turns out that Bones and Jody had done pre-pro testing on the KTM links and had a quiver of them that ranged from 3mm longer to 1.25mm longer. Eventually they settled on a 1.25mm link on the 250SX. This combination of parts brought the rear of the KTM 250SX down almost one inch.

I still had an issue with KTM’s seat foam. The stock foam breaks down quickly and gets spongy. My solution was to go into the MXA workshop and borrow seats off brand-new bikes.I didn’t make any friends with this strategy. In a last-ditch effort to stop me from stealing all the new seats, MXA ordered an Acerbis X-seat. This is a one-piece, molded plastic seat that doesn’t break down. It is very hard, but not as hard as the subframe rails. It retails for $199.95 and beats being followed around the MXA workshop by an armed guard.  For more info go to www.acerbis.com.

Step five: Sometimes you are in the right place at the right time?or somebody breaks their collarbone at the right time (sorry, Dad)?and that’s the way it was with the wheels on this KTM 250SX. MXA was testing billet TCR hubs and wheels, and I was the first to test them. There were some breakage issues with the first batch, but this has been worked out since then. TCR wheels are unique in that instead of making one hub that fits all the brands (with different wheel spacers), TCR makes purpose-built wheels for each brand. I got a brand-new set of orange anodized TCR KTM wheels laced with Excel spline-drive spokes to black DID Dirt Star LT-X rims. The two complete wheels cost $1300, but they were bulletproof. Plus, I still had the stock wheels to practice on. My choice for tires was Bridgestone 403/404s. These are my favorite tires for intermediate dirt.

Step six:
KTM is on the ball, and it shows in their attention to detail and the parts they spec on the KTM 250SX two-stroke. They made my job easy.

I didn’t need to add an oversize brake rotor to the front of the 250SX, because the bike comes with a 260mm oversize Brembo/Galfer combo stock. I didn’t need to add a Moto Tassinari VForce3 reed cage, because it comes with one stock.

I didn’t have to worry about clutch issues, because the KTM comes with a Magura hydraulic clutch?although I did switch to a stiffer KTM Belleville washer?which is a stock part on the 2013 KTM 450SXF Factory Edition. The 2012 KTM 250SX still has a coil spring clutch (in which we ran 450SXF clutch springs)

I didn’t have to buy a Twin Air filter, because it comes with one stock. (Plus, you don’t have to remove the seat to change the filter.)

I didn’t have to add any titanium parts to save weight, because at 217 pounds, the 250SX is 14 pounds lighter than a 250SXF, 20 pounds lighter than a 350SXF and 22 pounds lighter than a 450SXF. My main goal was to not add any extra weight that couldn’t be justified as a performance gain. Lots of the exotic parts that riders put on their bikes look trick, but weigh more than what they contribute can offset.

So what did I change? Here is the short list:

     (1) Kreft adjustable power valve. I don’t think this is the be-all, end-all of performance products, but it allowed for minute-preload adjustments to the main power-valve spring, which controls at what rpm the power valve opens. There was a big difference between getting the settings right and getting them wrong. I thought they were on the conservative side in the stock position.

     (2) Supersprox rear sprocket.
I switched back and forth between the stock 50-tooth rear sprocket and a lower-geared 51. www.supersrpoxusa.com

     (3) Brake rotors. When I switched to TCR hubs, I bolted on EBC Control brake rotors (in the stock size).

     (4) White Knuckle grips. KTMs come stock with Renthal grips, but I switched to White Knuckle grips because I like a grip with a small tread pattern.

Step seven: You might think that I’m not the best judge of how great MXA‘s KTM 250SX is because I came directly from a 150SX, but I disagree. Two-stroke experience pays off, regardless of engine size and I have raced the 250 Pro class on a 250 four-stroke, 150 two-stroke and 125 two-stroke. When I switched to the 250SX I felt right at home. And, I finished 4th overall in the 250 Pro class at the 2012 World Two-Stroke Championships on a bike set up identical to this (right down to the Showa suspension). Sadly, I missed the 2013 Two-Stroke Championship with a still healing broken femur?from kart racing.

This bike is set up for me, but you can pick and choose from the aftermarket parts (carb mod, pipe, clutch and gearing) you want, don’t want or think you need. As for the Showa works suspension?I didn’t look a gift horse in the mouth. Would you?

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