The MXA wrecking crew has always wanted KTM to build a KTM 300SX motocross bike. KTM hasn’t necessarily refused to build a production 300SX; instead, KTM points to the 300XC-TPI as a potential starting point for us to get what we want. Unfortunately, we are well aware of the problems associated with turning a 300cc enduro bike into a motocross bike. A portion of the 300XC isn’t a good fit for motocross, especially the XC’s six-speed gearbox, which has much wider spacing in the usable gears and two gears (first and sixth) that aren’t used in motocross. And although we have had good luck with KTM’s Transfer Port Injected (TPI) fuel-injection system (all of the 2021 300XC off-road models are fuel injected), for motocross we prefer a good old-fashioned carburetor. And, there is the price to consider. The 2021 KTM 300XC retails for $10,199, which is $1800 more than the $8399 2021 KTM 250SX. The last time we tried to turn a KTM 300XC into a 300SX, we vowed to never do it again.
THE BEST WAY TO GET THE NON-EXISTENT KTM 300SX DREAM BIKE IS TO START WITH A KTM 250SX AND BUY THE KTM POWER PARTS 300CC ENGINE KIT.
The best way to get the non-existent KTM 300SX dream bike is to start with a KTM 250SX and buy the KTM Power Parts 300cc engine kit. At $956.99, the Power Parts kit is a basic bolt-on kit that includes a new cylinder, gaskets, piston, rings, cylinder head, black box, wrist pin, circlips and power valve. Yes, it will raise the price of your basic KTM 250SX to $9356, but that is $890 cheaper than starting with a KTM 300XC-TPI and still having to buy a KTM motocross gas tank and 19-inch rear wheel and deal with finding the proper gearing for the wide-ratio six-speed gearbox.
For anyone who wants a KTM 300SX, we recommend building up a 250SX with the 300cc engine kit instead of buying the KTM 300XC or XC-TPI enduro model and tearing it down. We have built a lot of KTM/Husqvarna 300cc two-stroke race bikes by ourselves, but some of our best 300s have been commissioned from Husky/KTM guru Dick Wilk of Dicks Racing in Washington, Utah. Dick was in the KTM/Husqvarna business long before they joined forces, and he knows how to make the engine sing and the suspension hum. We have never been disappointed with a Dicks Racing engine or MX1 suspension re-valve.
DICKS RACING KTM 300SX ENGINE MODS
Dick Wilk is a creative thinker, and, best of all, he uses all that creativity in building his engine specs. Here is what Dick did to MXA’s test bike.
Porting. The port timing was raised 1mm. As you know, the higher the port height the higher the engine revs, which extends the range in which the engine continues to make power. It’s no secret that the KTM/Husky 300s have really strong low-end but sign off early. Raising the port timing allows the Dicks engine to rev. Of course, you can’t just raise the ports 1mm and call it a day or you will lose low-end.
Cylinder head. Dick machines the cylinder head to compensate for the loss of low-end caused by the higher port heights. Dick machines the cylinder head by 1mm to increase the compression ratio over the stock head. The increased compression makes the KTM 300SX engine hit harder and pull quicker into the midrange. Depending on how much Dick increases the compression ratio, the octane requirements of the fuel will go up accordingly.
Torque grooves. Dick adds his proprietary torque grooves in a ring around the head’s squish band to improve combustion efficiency. Dick claims the torque grooves funnel the misted fuel towards the center of the combustion chamber. The three pairs of grooves are aimed directly at each other; the fuel is completely atomized before it is ignited. According to Dick Wilk, this cylinder head machining improves torque and fights detonation.
Carb. Dick says that the stock 38mm Mikuni carb, once modified, can be made to work very well. For this build, we chose to replace the Mikuni with a 36mm Keihin. The smaller venturi of the Keihin increases the fuel’s velocity through the intake tract. Sounds good, but there is a glitch. The small carb throat and higher velocity result in a lower overall flow rate. As you would expect, Dick Wilk has an innovative fix for this. Using a proprietary machining process, Dick keeps the high fuel velocity of the 36mm Keihin carb while maintaining the high fuel flow rate of the larger 38mm Mikuni carb. How? From zero to one-third throttle opening, the Keihin carb’s venturi is 36mm, then from one-third throttle to wide open, the venturi is machined to be a 39mm carburetor. The effect is like having two carbs in one. One of Dick’s tricks to keep fuel velocity up as the carb increases in size is to install a Quad Flow Torque Wing between the slide and the reed valve to achieve a smooth, uninterrupted flow of laminar air over the contour of a wing, reducing surface friction drag. This simple little winglet looks strange, but MXA has enough experience with it to know that it helps increase air speed and torque.
Pipe. For an expansion chamber and silencer, Dick chose an Italian-made Scalvini cone pipe and silencer for improved top-end power.
Engine costs. Dicks High Performance 300 kit retails for $1195, fully assembled, modified and ready to bolt-on. If you stick with the stock 38mm Mikuni carb, the torque wing mods cost $225.00 A complete replacement 36mm Keihin carb, taper-bored from 36mm to 39mm (with the Torque Wing mod), is $750.00. The Scalvini cone pipe is $379.00. Scalvini also makes stamped KTM 250/300 exhaust pipes for $259.00. The Scalvini muffler costs $139.00 (aluminum), $209.00 (aluminum/carbon end cap) and $315.00 (full carbon fiber).
On the track, the Dicks Racing powerband was as fluid as it could possibly be. Don’t get us wrong, this was still an ultra-strong 300cc two-stroke, but because it revved instead of signing off, the powerband was spread out across a much wider range. Things that had to happen in a hurry on the stock 300 engine now carried on considerably higher in the rev range. With the combination of the increased top-end and the boost of higher compression on the low-end, this engine delivered 4.2 more horses than the stock KTM 250SX. Peak horsepower was 53.2 horsepower on the dyno, and at no point on the rpm curve did the 250SX come within 2 horsepower of the Dicks Racing 300SX kit. It was less of a big-bore feel than a big-power feel.
DICKS RACING/MX1 SUSPENSION MODS
As good as the 2021 WP XACT air forks are on the 2021 KTM 250SX, they can still be plusher. The XACT air forks use air pressure in the left fork leg to take the place of the typical coil spring. Conversely, the right WP fork leg holds the damper rod, mid-valve, shim stacks and the majority of the fork oil (there is some oil in the air leg for lubrication of the fork leg’s mechanical parts). For 2021, WP totally redesigned its damping system to effectively bleed off the damping spikes caused by oil and air constrictions. The stock WP XACT design works but is limited by the fact that one fork leg is holding the front of the bike up while the other is doing all the damping. When you calculate the 130 psi to 150 psi of pressure that it takes to hold the forks up, it takes a lot of force to get the forks to move over braking and chatter bumps. The air pressure causes harshness at the beginning of the stroke and then ramps up quickly at the end of the stroke to create too much resistance to free movement. Over the years, Dick has developed his Advanced Progressive System (APS) bypass technology to blend the fork’s travel into a single, seamless flow. Instead of being too harsh at the beginning of the stroke or too soft at the end of the stroke, Dick Wilk’s APS system constantly blows off excess pressure as the fork moves through its stroke. It worked well on WP’s older model forks, but Dick knew that there had to be a better way.
The better way is his MX1 Hybrid AER fork system. The Hybrid AER design eliminates air pressure’s chronic stiction and ramp-up issues by allowing the fork pressure to be reduced to 60 psi in the left air leg and combined with a new damper cartridge that works in conjunction with a .52 kg coil spring in the right leg. The coil spring does the majority of the work when the forks move initially, while the low air pressure kicks in later in the stroke when it is compressed. In essence, the fork legs split the workload—one for small bumps and one for big hits. Best of all, they maintain the integrity of the WP XACT concept by reducing pressure spikes from both air and oil. The MX1 Hybrid AER fork kit retails for $695 (2021-2022 WP XACT forks) and $795 (on 2017 to 2020 forks). The MX1 Hybrid fork mod is much cheaper than most fork re-valves, because MX1 focuses most of its efforts on one fork leg instead of two.
As for the rear WP shock, it was re-valved to match the flow characteristics of the dual-stage action of the forks. The stock 4.2 kg/mm shock spring was replaced with a 4.5 kg/mm spring to accommodate a 190-pound rider. Static sag was set at 35mm, which equated to 105mm of rider sag. We set the shock’s low-speed compression clicker on 20 clicks out, the high-speed compression on two turns out and the rebound on 15 clicks out. The MX1 shock re-valve costs $210.
ON THE TRACK, THE DICKS RACING POWERBAND WAS AS FLUID AS IT COULD POSSIBLY BE. DON’T GET US WRONG, THIS WAS STILL AN ULTRA-STRONG 300CC TWO-STROKE
All the MXA test riders loved this project bike. It was unique and, unlike our last couple KTM 300SX motocross projects, this one didn’t cost us a ski-boat budget. But, it wasn’t cheap. With the stock Mikuni carb, we could have done the complete engine, exhaust and suspension package for $3710. The Keihin carb package added $525 for a total cost of $4240. For more information, contact Dicks Racing at (916) 705-3193 or www.dicksracing.com.