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Reshaped combustion chamber has higher compression for harder hitting low- and mid-range response.
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Revised porting for the composite plated cylinder delivers increased performance across the rev range. (Reshaped intake ports and elimination of the
intake sub-ports contribute to low- and mid-range performance; revised exhaust and scavenge port timing increases mid- and top-end performance.) (Photo 2 image shows inserts to indicate final
change)
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The 2-stage 3-way KIPS has been further improved. A new 1-piece aluminium KIPS valve replaces the earlier 2-piece steel valve, resulting in improved
sealing between the exhaust port and valve, and crisper low- and mid-range throttle response.
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Additional improvements to the KIPS include a reshaped governor guide and a revised load for the governor spring. These changes contribute to
improved low- and mid-range power.
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New expansion chamber dimensions contribute to improved power in the mid-range and at high rpm.
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New settings for the carburettor complement the other changes and deliver crisper response at partial throttle openings.
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The shape of the air duct leading from the airbox to the carburettor has been reconfigured to allow straighter airflow. This further contributes to
the improved throttle response.
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The carb holder has been re-angled for straighter airflow, which also contributes to enhanced response.
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New crankcase castings have eliminated the boost port at the rear of the cylinder. This modification speeds up the flow of fuel/air through the
transfer ports for improved low- and mid-range power characteristics.
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The crankcase “stuffers” mounted to the crank webs have been changed from aluminium to nylon. The revised inertial balance (through elimination of
lead weights) of the crank also contributes to the sharper throttle response of the ’04 engine.
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Crankshaft oil seal pressure has been reduced by using a smaller diameter collar on the crank journal, resulting in less friction and a slight
increase in throttle response.
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New ignition timing and new ignition rotor improve the linearity of low- and midrange power.
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Breather added to KIPS cover eliminates the influence of exhaust pressure on the KIPS valve for more precise action.
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Piston durability and resistance to seizing have been improved by machining the piston skirt with microscopic oil-retaining grooves and by plating
the piston with a very thin coating of tin.
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The kick starter has been improved. A bushing on the pivot reduces wear and subsequent free play, and a steel pin pressed into the stopper prevents
the stopper area from wearing and the kick start lever from angling out excessively.
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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.
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The rear brake pedal is now mounted 11.5 mm higher for increased ground clearance, and to suit the new riding position.
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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.
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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.
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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.
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As before, the aluminium steering stem is tapered for low weight.
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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.
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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).
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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.
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Relocated muffler bracket is more durable.
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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 2 mm longer (max. length increased from 604.5 mm to 606.5 mm).
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The sub-frame benefits from a stronger silencer mounting bracket and a smaller side cover mounting bracket.
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Rear frame pop nut has reduced height for lower weight.
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New engine mounting bolts made of stronger material can be torqued tighter and stay tight better.
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A stronger igniter bracket is fitted.
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Front brake line is now routed directly rather than under the calliper as on the ’03 model, resulting in a slightly lighter brake hose.
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Dual-piston front brake calliper uses 27 mm pistons and is operated by a master cylinder with an 11 mm piston.
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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.
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The front axle is 9 mm shorter and the axle collar is also shorter and lighter (both are common with the KX250N).
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A stronger chain guard of more durable material is also fitted.
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1-piece aluminium front and rear wheel spacers for low weight.
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Fuel tank now uses an oblong mounting hole for easier maintenance.
