The KTM 500EXC is a ridiculous machine, and not just because it’s plate legal. Engineered into its DNA is some rare strain that gives this wagon the ability to bite into the off-road world with a fairly hearty appetite. As a legal tarmac traveler with its alter ego a very legitimate off-road persona, the 500 is without a doubt the best do-it-all legal machine made. But serious aficionados will never leave it alone and will push the limits of performance, competitiveness and legality to the penultimate edge. So instead of making you guess at what works and what doesn’t, we’re going to give you suggestions that will strip the machine of its plate legality—at least in California—but will enhance its craving for chewing on uglier and more brutal terrain. A big thanks goes to FMF—whose modified machine we tested, compared to our stocker—who gave us hoards of tech and setup information.

First off, if you want the bike to remain street-legal—at least California legal—you cannot modify or adjust the fuel injection, the emissions or the exhaust. That’s the bottom line. You can modify it to death if closed-course riding is your goal, and in that case, getting more boost out of the smooth, lean-running 500 is mandatory. There are several steps for dealing with the emissions. (Remember, this is only if you want to motocross your EXC.)

Gearing: You have two choices here. Most people opt to change the stock 15-45 gearing to 14-48. This is because 14-48 is pretty good, and with this, you don’t have to buy a new longer chain. However, 14-49 or 14-50 is probably better if you can stomach the cost of buying a new longer chain and rear sprocket. First, it’s much better for tight stuff, as you can stay in second gear more—rather than push first or totally lug second to the point of flaming out. If you think you’ll run out of speed, you won’t. Peak power is at 8000 rpm, and the bike goes to 10,000 rpm. With 14-50 at 75 mph on the street, you’re only hitting 7000 rpm. The second reason is that the wheel can stay back in the axle slot if you use a 117-link chain to fit the 50-tooth rear. This keeps the rear suspension more supple in the small bumps.

Get rid of the popping (on decel): First, plugging the vent line to the air pump effectively richens the circuit slightly and lets the machine run better down low. Go to your hardware store and get a rubber- or cork-tapered plug that’s 5/8-inch on the small end. Then, pull the large rubber line from the exhaust air valve on the right side of the cylinder and jam the plug in it. Put the line back on the valve. This prevents the valve from sucking air and rids you of the pop most of the time. This is not street-legal.
Quits running or starts shooting ducks on long downhills: Since the bike is EPA-friendly, the fuel overflow and vapors must be contained in the vapor canister. This is the black plastic canister under the right side of the gas tank. On sustained (and steep) downhills, the fuel runs into the vapor system and eventually creates vapor lock. The easiest fix is to not overfill your gas tank—leave about 2 inches from the top—or pop the vent line off the gas cap if the machine starts acting up. A closed-course fix is to remove the gas tank and snip the line where it runs under the tank, next to the ignition coil.
More power: We tested a No Loss throttle body (www.nolosssthrottle, and this was perhaps the strongest performance mod that we made. Because the 500EXC is both EPA and CARB legal, it runs pretty lean around 1/3 throttle, where their tests are performed. The computer on the bike is actually locked, which means you can’t go in there and reprogram it like you can on the off-road models (to alter the ignition mapping and fuel). If you try to disable the emission system on the bike, the computer knows and the bike won’t run right.
The No Loss throttle body replaces the stock Keihin and uses a plate that pivots rather than a butterfly valve to control air and fuel flow. This leaves the throat wide open with no restrictions, filling the cylinder with a high-velocity stream of air. It’s virtually friction-free, pivots on needle bearings, and will not bind on high-vacuum engines. The No Loss richens up the lean settings in the stock ECU by manipulating the airflow at different throttle openings. Because the bike runs lean in certain spots and the computer is locked, No Loss uses special plate shapes to regulate the amount of air at the different throttle openings. This results in a better air/fuel curve than you could ever get with the stock throttle body. On top of this unique feature, the No Loss moves a lot of air at high velocity. There are no additional mapping requirements, though the system is tunable both at the ignition timing and mixture settings. The biggest difference is that it outflows the stock unit, and this equals a big—maybe even huge—power gain. On the 500EXC, it actually hurt its off-road palatability because the soft roll-on is replaced with an immediate and intense throb of power. It pulled longer and substantially harder, changing the machine from a trail-friendly bike to a brute that craves monster hill climbs and rips horrendous wheelies in loam that would bog down a John Deere tractor. For moto, closed-course power-speed courses, deep sand tracks and hills, it’s a giant gain. This is a closed-course-only modification.
Exhaust mods: The stock exhaust is new for 2012 and uses a screen-type spark arrestor with a flow tube that stifles the exhaust evacuation, therefore reducing the decibels over this same unit on an XCW. By removing the outermost tube, you get a substantial power gain and a minimal exhaust-note rise.
More power, less weight—FMF MegaBomb and 4.1 RCT with spark arrestor: If you just switch the stock muffler to the 4.1 RCT with the spark arrestor insert, you’ll get a really nice increase in power. The bottom end goes up a bit, but the real improvement is in the mid to top end, which is exactly where the 500 is a bit shallow. The RCT is exactly 1.1 pounds lighter than stock and is stuffed with expensive packing. It will let you log some serious miles before the packing evaporates. The unit is louder than stock, but it is a quiet system with the SA insert. It is closed-course legal (not EPA or CARB legal).
One of the best ways to add bottom and mid power and make it quieter is the stainless steel MegaBomb header. FMF ( invented this technology, and because it staggers the exhaust waves, two things happen: volume decreases and flow increases. We’ve found that the MegaBomb works really well with quiet rear systems.
Precision Concepts suspension: The stock WP dampers are quite decent, though valved and sprung light. For causal trail riding, both ends are fine, accepting trail litter, rocks and roots with aplomb. Faster obstacles require a thoughtful eye, and when you plop a 185-pound-plus rider on top, things are squished and the handling gets quick and loose. The cheapest fix is stiffer springs, both fore and aft. This works well for most trail and enduro conditions for riders topping the buck 80 number. For pilots who want to test their talents against the terrain, we heartily suggest Precision Concepts’ (www.pre fork and shock mods. We wanted our bike to be more adept at speed (for the occasional GP or moto foray), but didn’t want it too harsh for rocky trail carnage. Precision Concepts gave us exactly what we wanted. The front open-cartridge fork received suspension valving changes, plus an increase in spring rates. Similar updates were made to the rear WP damper. The suspension went from salivating on trail gruel and wincing on speed obstacles to acceptable behavior on slow-speed rocks and roots and superb manners on mid-speed hits, jumps and high-speed desert whoop sections. Besides the increase in its ability to attack terrain, rather than grimace and hope, the bike’s overall cornering and handling traits went up several notches. The bike sat up and was balanced when cornering, held a line better (since it wasn’t diving dramatically) and seemed to make better traction at both ends.
GPR for stability: With a new chassis, soft suspension and improved ergos, the new KTM feels short, quick and whippy. Stiffening up the suspension helps dramatically, but one of the best mods we made was installing a GPR V4 ( steering damper. We used their Fat Bar kit mounted in the top clamp’s front bolt holes (for the most room) and the bars raised about 3/8-inch. The new GPRs have an improved feel—they feel thinner—and offer more range at the adjuster. Still, it nearly always ran best at the softest #1 setting or #2 on fast, desert terrain. This lessened the muddy feel at slower speeds, seemed to work really well with the stock soft suspension, and straightened out a ship that liked to dance in rough and rocky terrain.
Doubletake mirror: The stock rearview mirror is fine for the street but doesn’t cut it in the dirt. For off-road use, you must have a foldable unit that can tuck out of the way. The Doubletake Mirror (www.double is the first that actually makes sense as a road mirror and a get-it-out-of-the-way mirror when the going gets dirty. It is made from quality material, offers easy-to-manage position changes, and is available for screw in mounts or can be mounted straight to the handlebar.


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