Once running, the new RF4 engine has a snap and crispness to it that make the old SOHC RFS motor seem, well, enduro. Just like the 250 SX-F but one step further. There is a little mechanical whine-blame the spinning gears. The 41mm Keihin carb was jetted spot-on for warm temperatures and for altitudes up to 4000 feet. Yet with all this talk of the improved power, KTM didn't forget to make the motor smooth. Even when boasting extremely high peak-horsepower numbers (which we believe), it feels like the numbers on the bottom are in line, if not a little less than some of the other bikes. The bottom and midrange are linear and progressive, with plenty of torque to pull a gear high or lug around the track. You can ride in the low rpm range and use big throttle openings without the consequence of bog or the bike ripping out of your hands. Run it up onto the top and the bike really begins to come alive, as there is such a big surge and pull that hitting the rev-limiter is pretty rare. In fact, we found the biggest shortcoming of the four-speed gearbox was rough high-speed straights when you were pinned. The bike wanted to lift the front wheel when you'd have shifted and run at a lower rpm (and less power) on a five-speed bike. Inside the ignition box, there is a second curve-less aggressive on the top-end, activated by disconnecting a wire or through a KTM Hard Parts switch. We tried it and didn't notice much difference. There isn't a problem pulling the gaps as the gear ratios are typically what second through fifth would be, just geared lower in the final drive to shift down the ratios. A couple things are for sure: This bike revs, is willing to rev and makes boatloads of power up top for the rider twisting the throttle. It will surely be an easy transition for a 250F rider.
The clutch pull isn't the lightest, but it does get the job done when stretching those tight corners into a third-gear job. And it has good, biting engagement. At the majority of tracks we tested on, the bike worked mostly in third and fourth, prompting us to try a 51-tooth rear sprocket, which it pulled easily. We think on faster tracks a 50-tooth isn't out of the question. As when riding most four-speeds, have sprockets handy. Shifting was solid the whole time we were on the bike, from full-throttle upshifts to bringing it down into the turns. And the compression braking on a single downshift wasn't an issue, just some good trail braking, but double downshifts before slowing are a little harsh.
The bike has a slightly tall feel the first moments you are on it. Riders were split between whether the bike felt long or short, but it is way more along the lines of the four Japanese brands than in the past. The riding position is flatter than it was previously, both leading up onto the tank and getting back on the rear of the bike, making it easy to move around on. The tank, sidepanels and shrouds are never too wide and grip well, especially standing. The chassis seemed to fit most riders, but we had taller guys who thought it was plenty roomy and some shorter riders who felt a little cramped. The distance from the footpeg to the seat top is approximately 11/42 inch shorter than on most bikes, which could account for those with longer legs feeling the pinch. There is always the option of moving the handlebar forward, as ours was in the second position for the duration of the test.
Suspensionwise, the KTM also makes headway, first in plushness, especially once the fork breaks in, then in bottoming resistance, especially in the rear compared with older SXs. Now using a lighter spring and more valving control, the shock might not span the wide range of rider weights it used to, but once dialed, it gets the job done. For our riders ranging between 160 and 185 pounds, we set the bike-only sag at 35mm (which translated to 107-115mm with rider) and were pretty pleased. We ended up going a couple of clicks stiffer on low-speed compression and added one-eighth and one-sixteenth turn of high-speed compression. This actually held the bike higher and made the shock plusher in the initial travel. A stiffer 6.9kg/mm spring makes a positive improvement for riders above 185 pounds, keeping the shock from packing in the midstroke. Ditto for the front, go stiffer, in most cases two to six clicks depending on the track. The bike really responds to fork tuning, and it is real easy to dial it in, particularly the character of the ride in the turns. On hard-packed turns, tightening (slowing) the fork rebound damping held the bike better in turns.