MXA TWO-STROKE FILES: WE RIDE FMF’S KX500/KTM 250SX HYBRID
There is a recurring dream that persists in the motocross psyche — the dream of building the ultimate racing weapon. Lots of lunch hours and bench-racing sessions are filled with discussions about building the perfect motocross bike. The conversation always focuses on finding a chassis that is light, narrow and compact. The frame has to be sourced from readily available units, and, most important, the ultimate race bike needs the most powerful, yet manageable, powerband that the sport’s engineers have ever produced.
One important caveat in the dream-bike scenario is that the bike cannot have a 450cc four-stroke engine. Why would it? Four-strokes are readily available and thus not dreamy enough. That means the ultimate motocross bike would most likely be powered by a 500cc two-stroke engine. It is the rarest of engines, and it makes the most torque and competitive horsepower. It can be detuned or hopped-up with ease, and it is lighter than competing 450cc four-stroke engines.
A FEW YEARS AGO, THE FIRST CHOICE WOULD HAVE BEEN A CRF OR KX250F FOUR-STROKE FRAME. IT WAS CONSIDERED THE EPITOME OF MODERN DESIGN.
Okay, with the basics of the dream bike down on paper, it is time to make some decisions about the frame and engine package.
(1) Frame. The first hard decision is choosing between an aluminum Delta-Box frame from an existing four-stroke and a chromoly steel KTM chassis. A few years ago, the first choice would have been a CRF or KX250F four-stroke frame. They were considered the epitome of modern design. But, as of late, KTM’s oval-tube, chromoly steel frame has begun to overshadow the rigid Delta-Box design. Not only does KTM’s steel frame handle very well, it has more room for the engine of choice and can be cut, chopped and welded with ease. Decision made: 2013 KTM 250SX frame.
(2) Powerplant. Choosing a 500cc two-stroke engine is as simple as ordering Chinese food. You select one from column A or one from column B. In column A is the venerable Honda CR500 engine, and in column B is the equally venerable Kawasaki KX500 engine. The Honda CR500 engine was last produced in 2001, but parts are readily available, and it has a great reputation left over from its days as the engine of choice of Jeff Stanton, Jean-Michel Bayle, David Bailey and Rick Johnson. On the other hand, the last KX500 engine rolled off the assembly line in 2004 and is actually a better all-around powerplant than the CR500 engine. It has a broader powerband and more controllable output. The reason it gets less publicity than the Honda CR500 engine is because the KX500’s suspension, layout and ergos were less suited to motocross than the Honda’s. Decision made: 2004 Kawasaki KX500 engine.
Not every rider has the same dream, but the MXA wrecking crew ran into two industry powerhouses who did share the same vision and were doers more than dreamers. Four years ago, FMF’s Donny Emler and former 250 World Champion Danny LaPorte had the same conversation that we just discussed. And, unlike most bench racers, they decided to make the dream a reality. Danny had raced the Baja 500 for Team Kawasaki and believed that a KX500 engine could still win Baja if it were in the right chassis. Donny and Danny decided to go with a used 2013 KTM 250SX chassis, plus its suspension, running gear, wheels and brakes. They chose it because they liked the way it handled, and they liked that the chromoly steel could absorb some of the heavy vibes of the KX500 powerplant.
IT DIDN’T HURT THAT DONNY EMLER HAD BUILT HIS REPUTATION BACK IN THE GLORY DAYS BY SWAPPING OUT HONDA AND PENTON PARTS TO BUILD GREAT 125cc RACE BIKES.
Once they chose the engine and the chassis, the daunting task of putting the two together began, except that it wasn’t a daunting task. Danny LaPorte says, “When we first looked at the Kawasaki engine and the KTM frame, we didn’t think it would fit in there, but we were wrong. The first time we set our rebuilt KX500 engine into the KTM frame, we realized it was going to be the easiest engine swap ever.” It didn’t hurt that Danny’s partner on the project, Donny Emler, had built his reputation back in the glory days by swapping out Honda and Penton parts to build great 125cc race bikes.
PUTTING THE KX500 ENGINE INTO THE KTM 250SX FRAME WAS SIMPLE (FOR THESE GUYS)? JUST A THREE-STEP PROCESS.
Step one: To get the KTM swingarm pivot bolt through the Kawasaki KX500 engine cases, the cases were center-bored. Emler elected to bore the cases just a few thousandths larger than the KTM pivot bolt to get as snug a fit as possible. The swingarm pivot bolt is the only fixed position on the engine swap. To line up the countershaft sprocket and rear KTM sprocket, a 6mm spacer was machined to push the engine over to the side. This was a simple task, because the KX500 engine cases were narrower than the space between the KTM frame’s down rails.
Step two: To get the crank as low as possible, Donny and Danny decided to do whatever was necessary to get the engine cases as low as possible in the KTM frame. To accomplish this, they notched the frame cradle tubes under the engine and welded chromoly tubes into the notches to accept the bottom motor-mount bolts. They went as low as they could go and still have tubing left under the frame cradle, but the actual position was determined by simple logic. They put the bottom motor-mount bolts in the spot that made the embossed Kawasaki logo on the clutch cover level. Additionally, they had to dent one frame cradle tube to get access to an engine-case bolt.
Step three: The final task was to fabricate new front motor-mount plates and a head stay. The rest of the build was basic problem-solving for the coil, intake tract, reed valve and shift lever. Danny and Donny insist that the engine swap was easy as pie and, by engine-swap standards, this is true. But, no mechanic ever tells you the whole story. They conveniently forget their frustrations and head scratchers.
IT WAS REALLY DANNY LAPORTE WHO PUSHED FOR THE KAWASAKI POWERPLANT, BUT HE WASN’T LOOKING FOR MORE POWER.
It was really Danny LaPorte who pushed for the Kawasaki powerplant, but he wasn’t looking for more power; he knew that it would have more than enough in stock trim. Instead, the duo tried to detune the KX500 a little bit with an intake air boot that had a little less volume for better throttle response and a touch less top end. The compression ratio was left stock, as was the 40mm carburetor.
Getting an exhaust pipe built was no problem, because Danny LaPorte knew a guy who owned a pipe shop. Luckily, FMF builds exhaust systems for the Service Honda CR500AF aluminum-framed bikes. So, Donny started with some CR500 cones, custom-rolled a few more, and made a very trick lowboy KX500 pipe that fit like a glove. The only problem, and it wasn’t a small one, was that the water spigot on the right side of the cylinder was directly in the path of the pipe. Now, you might think that a famous pipe builder would just weld up a new pipe to work around the spigot. Not so! Instead, Donny moved the water spigot over 2 inches and covered up the old spigot hole. Problem solved. For a muffler, Donny selected a KTM PowerCore silencer, because it would mount right up to the KTM 250SX subframe.
Although you wouldn’t know it by the looks of it, this bike was designed to race the Baja 500. But, a funny thing happened on the way to Mexico. Danny started riding it at local motocross tracks to shake out the bugs, and the more he rode it, the more he liked it. So, Donny and Danny decided to use it as a motocross bike and start on another KTM/KX500 hybrid for Baja.
JODY AND DONNY TALKED ABOUT THE BIKE FOR A FEW MINUTES, AND THE NEXT THING YOU KNOW, THE BIKE TRADED HANDS. DONNY GAVE HIS PROJECT BIKE TO MXA ON THE SPOT.
This is where the MXA wrecking crew came in. We were parked next to Danny and Donny at Glen Helen on a day when they were shaking down their project bike. Jody and Donny talked about the bike for a few minutes, and the next thing you know, the bike traded hands. Donny gave his project bike to MXA on the spot. Jody sent three different MXA test riders out on it immediately to iron out some suspension and jetting issues. From that moment on, MXA started racing the KTM/KX500 every weekend. Here is what we learned (and we kept Donny’s KX500/KTM for almost four months, raced it at the World Two-Stroke Championships and used it to scare young guys).
You might think that a 500cc two-stroke engine would be brutally powerful, or that it would loop over backwards with the slightest twist of your wrist. You might also be led to believe by the current crop of pantywaists in the sport that you can go faster on a 450 four-stroke than a 500 two-stroke. Not true. The KTM/KX500 makes a lot of power, but it puts that power to the ground in such a way that even a Novice could go fast on it. Really fast! It is true that you could get into trouble if you got carried away with yourself, but on long straights, wet loam, deep sand or big uphills, nothing can touch a 500cc smoker.
When you ride the KX500 properly, it feels like it has an automatic transmission. The MXA test riders didn’t have to do very much shifting, and clutch action was limited to pulling it in when overshooting corners (because they misjudged the speed they were traveling). In many ways, the KX500’s broad, tractable and automatic powerband (it is a five-speed) allowed the riders to concentrate on picking lines, grabbing big handfuls of brake and lining up the competition on the straights.
Since the engine was stock, we did most of the tuning with the rear sprocket. Our Pro test riders wanted it geared down one tooth so they could get to third gear sooner. Our Novice and Vet test riders didn’t like the lower gearing because it made the KTM/KX500 more abrupt in second gear. There were even some test riders who preferred to gear it up to make each gear mellower and longer.
AS THE YEARS GO BY, IT IS HARDER TO FIND TEST RIDERS WITH ANY 500cc EXPERIENCE. LUCKILY, MXA KNOWS A BUNCH OF FORMER NATIONAL PROS WHO KNOW HOW TO USE A 500cc TWO-STROKE TO ITS FULLEST.
As the years go by, it is harder to find test riders with any 500cc experience. Luckily, MXA knows a bunch of former National Pros who know how to use a 500cc two-stroke to its fullest. Compared to a Honda CR500, the Kawasaki KX500 engine is hands down faster. The best way to control the power is with a series of short-shifts to keep the engine in the meat of the powerband.
Every MXA test rider loved the KTM/KX500. For the younger test riders, which was any rider under the age of 30, this was their first experience with a 500cc two-stroke. They struggled with the power in the beginning, but once they learned that you didn’t need to use all the power the bike produced, they began to get in tune with the big beast of an engine. It came on harder, pulled harder and required a moment’s thought before pulling the trigger. A 450 four-stroke might be intimidating for some, but the KX500 powerplant produces a whole new level of terror in the untalented or unwary.
Overall, we loved the KTM frame. The bike handled like a dream. The brakes were awesome, and the layout was rider-friendly. Donny installed an aftermarket Magura hydraulic clutch to help the KX500 clutch pull. Since the KX500 engine was heavier than the KTM 250SX mill, the bike gained 13 pounds. But, even with the extra weight, it was still lighter than most 450 four-strokes and felt even lighter in motion. Donny had Factory Connection add stiffer fork and shock springs to compensate for the engine’s weight and power, but the WP forks and shock were left stock.
The truth is that if KTM built this bike, with a KTM 500cc engine in place of the Kawasaki powerplant, it would sell. But since we tested it, almost four years have past—and there have been no big-bore two-strokes coming down the assembly lines in Austria or Japan. That’s a shame because it’s a fun bike to ride, a blast to climb hills on and a guaranteed holeshot at any track in America. And, since it is as close to a production 500cc as anyone has ever come, it doesn’t have any compromises. It lives up to the motocrosser’s dream; it is light, narrow, compact and powerful. Oh yeah, by engine-swap standards, it was also easy to do. But, if you want one, like Danny and Donny did, you’ll have to do what Donny and Danny did. Use their end reult as a blueprint.