Engine Improvements

• Reshaped combustion chamber raises the compression for increased low- and midrange output, and crisper throttle response.

• New piston ring reduces ring pressure (from 5.6 N·m to 3.8 N·m), for improved throttle response.

• Improved KIPS valve design reduces the gap between the valve and port when the valve is fully closed for improved low-rpm power.

• Expansion chamber redesigned for more power in the mid-range and on top-end.

• New 2-stage reed valves use carbon fibre reeds as last year, however, the weave direction has been rotated to a 45° angle. By increasing the flexibility of the reeds, low- and mid-range torque characteristics are improved. To prevent over-flexing of the reeds at high rpm and to stiffen them, a second layer of smaller, glass fibre reeds (also with 45° weave pattern) reinforce the main reeds. The result is the best of both worlds: highly responsive low/mid-range with the performance of stiffer reeds at high rpm.

• New carb holder and airbox duct give a straighter flow of incoming air for improved throttle response at very-low and low-rpm.

• A reduction in diameter of the primary drive gear’s oil seal contact area (from 32 mm to 31.4 mm) reduces frictional loss and contributes to the improved throttle response of the new engine.

• Revised ignition timing further boosts performance and improves reliability by reducing irregular combustion.

• A new cylinder features a wider radius at the upper corners of the exhaust port rib to reduce piston friction and improve piston and piston ring durability.

• New cylinder base gasket is now symmetrical for easier maintenance.

• Cylinder cover changed from plastic to aluminium for improved sealing.

• A new piston relocates the piston ring locating pin at the exact rear of the piston. This ensures even heating/expansion of the piston ring and improves reliability of both the pin and the ring.

• With the exception of new settings, the TMX38x carburettor with arch-shaped slide remains unchanged. This unusual slide shape helps to concentrate the incoming charge more towards the centre of the intake tract for increased power in the higher rpm ranges. Also fitted is a semi-primary bleed needle jet that homogenises the fuel air mixture, contributing to sharp throttle response across the rev range.

• New crankshaft features increased clearance between the connecting rod big-end and the nylon crankcase “stuffer” for better lubrication of the big-end bearing. New connecting rod of stronger steel is more durable and improves reliability.

• A new casting for the left crankcase is more rigid and prevents oil from seeping out between the cases. Additionally, the shape of the clutch cable hole has been changed to improve cable-to-case sealing (via an O-ring used on the cable instead of the previous bushing), and the breather hole has been relocated to prevent transmission oil from blowing out.

• A slightly thicker generator cover gasket improves sealing.

Frame and Handling

• The most significant modification to the lightweight steel perimeter frame is the new link Uni-Trak system. Additionally, the thickness and shapes of the upper cross tubes and engine mounts have been modified to idealise the stiffness balance for enhanced cornering performance and improved durability. Further, the footpeg location is now 3 mm higher for greater ground clearance when banked over and to complement the improved riding position.

• The rear brake pedal is now mounted 11.5 mm higher for increased ground clearance, and to suit the new riding position.

• Front Suspension - New Kayaba bladder fork features check valves on the bladders to control their internal pressure. The result is improved damping and better fork action throughout the stroke. Other modifications to the 48 mm inverted fork include a longer interior slide bushing for improved action, a “waisted” interior cylinder for reduced weight, and more compact axle brackets with a new clamping method.

• Rear Suspension - The improved Uni-Trak rear suspension system uses a newly designed link and a new operating system. Unlike the ’03 chassis in which the link pivoted on the frame, in the new system, a newly designed link pivots on the swingarm. The advantages are two fold: first, the brunt of the suspension loads are now absorbed by the swingarm, allowing a lighter frame construction. The second benefit is that the shock absorber now moves in an outward arc as it compresses, rather than moving out and back in. This smoother, more stable action significantly improves traction.

• The swingarm is also new, and features a new cast bracket just aft of the shock to suit the new link, new forged axle holders (like those of the KX250N) which are stronger, and lighter, smaller diameter chain adjuster bolts (reduced from 10 mm to 8 mm). Plastic plugs now cover the holes in the swingarm beams to prevent the ingress of mud and dirt. The new swingarm features the tapering, hydroformed beams of the ’03 model but is about 20 mm longer (max. length increased from 579.5 mm to 596.5 mm).

• The sub-frame benefits from a stronger silencer mounting bracket and a smaller side cover mounting bracket.

• Rear frame pop nut has reduced height for lower weight.

• New engine mounting bolts made of stronger material can be torqued tighter and stay tight better.

• A smaller expansion chamber centre section contributes to the improved riding position by allowing the rider greater freedom of movement.

• While the thin-wall aluminium triple clamps remain unchanged, the top nut is now made of lightweight aluminium instead of steel, and the washer below it is a thin (1 mm) stainless steel piece instead of the ’03 model’s thicker mild steel washer.

• As before, the aluminium steering stem is tapered for low weight.

• The actuating rods for the Uni-Trak linkage are new. A one-piece forged rod replaces the ’03 model’s two-piece extrusions. Also new are the sleeve-rod bushings, which are now captured by stepped washers (the washers stay in place better). A new D-head rod bolt eases maintenance (no wrench needed).

• In addition to a new spring and revised damping settings, the rear shock rod length is 20 mm longer to suit the new link, and a drain bolt has been added to the shock for easy oil changing.

• Front brake line is now routed directly rather than under the calliper as on the ’03 model, resulting in a slightly lighter brake hose.

• Dual-piston front brake calliper uses 27 mm pistons and is operated by a master cylinder with an 11 mm piston.

• One-piece rear brake master cylinder is compact and lightweight and operates a 240 x 4 mm disc. Brake master cylinder rod length shorted 12 mm to suit new brake lever position.

• The front axle is 9 mm shorter and the axle collar is also shorter and lighter (both are common with the KX250N).

• Drive chain length increased from 112 to 114 links (now same as KX250).

• A stronger chain guard of more durable material is also fitted.

• 1-piece aluminium front and rear wheel spacers for low weight.

• Fuel tank now uses an oblong mounting hole for easier maintenance.