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motocrossactionmag.com

TWO-STROKE TUESDAY: SERVICE HONDA’S KX500AF TWO-STROKE

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What exactly is needed to make a Service Honda KX500AF roll out of the workshop? Starting with a 2009 Kawasaki KX250F chassis, the technicians tear down the bike to the bare frame and begin fabricating various areas of the frame. The engine cases are re-drilled, the swingarm spacers are changed, the head stay and head stay base are fabricated. Obviously the frame cradle has to be reworked severely. Service Honda cuts half of the cradle out, and the juncture of the frame under the gas tank is relocated for engine clearance. Additionally, the “Y” area of the frame is relocated three to four inches forward in order to clear the exhaust pipe. The same grade of aluminum alloy is used on the rebuilt section of frame as on the stock frame to give the KX500AF the stock look.

A.J. made a point of telling us that reworking the radiators was the most tedious part of building the bike. The radiators alone take a five hour chunk of time out of the entire process. The stock KX250F radiators were used because they have a greater surface area than the stock KX500 radiators. A.J. has the tanks and the hose barbs reworked. It was interesting to find out that the left side radiator was flipped over to become the right side radiator. Why? The KX250F left radiator was too long and didn’t provide enough room for the FMF-built KX500AF exhaust.

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TEST RIDE: TRYING TO TAME THE BEAST

2009 KX500 AFWith the advent of new technology and a revised approach to the engine characteristics of four-strokes, the smooth and rider-friendly four-stroke powerband trumped the 500cc engine (not in pure speed, but in usability).

The MXA wrecking crew has included a number of seasoned 500cc two-stroke riders (and we tested with former 500 National Champion Chuck Sun, Yamaha of Troy and former National rider Phil Alderton, Willy Musgrave and a host of other MXA test riders) all who cut their teeth years ago riding the Open bikes. These test riders remember what it felt like to use a 500cc engine to its full advantage. Of course the younger MXA test riders couldn’t fathom the techniques needed to ride the Kawasaki KX500AF fast. The whippersnappers have been spoiled by the new-generation 450cc four-stroke engines.

ON THE TRACK, THE SERVICE HONDA KX500AF ENGINE RIPPED. IN COMPARISON TO THE HONDA CR500AF THAT WE TESTED LAST YEAR, THE KAWASAKI ENGINE WAS HANDS DOWN FASTER. OF COURSE THIS DIDN’T COME AS A SHOCK TO OLDER MXA TEST RIDERS. WHEN THE TWO MOTORCYCLES WERE STILL IN PRODUCTION, THE KX500 ENGINE TRUMPED THE CR500 POWERPLANT

On the track, the Service Honda KX500AF engine ripped. In comparison to the Honda CR500AF that we tested, the Kawasaki engine was hands down faster. Of course this didn’t come as a shock to older MXA test riders. When the two motorcycles were still in production, the KX500 engine trumped the CR500 powerplant and was preferred by every MXA tester (they just preferred the CR500 chassis over the KX500 chassis). To control the power of the KX500, the best technique is a series of short shifts to keep the engine in the meat of the powerband. The 14/47 gearing combination worked well at maintaining drive, and the FMF exhaust and silencer were a nice addition to the engine profile.

The million-dollar question is which is better, a modern KX500 or a modern four-stroke? In a head-to-head comparison, the 500cc two-stroke engine was much more difficult to ride. It came on harder, pulled harder and required a moment’s thought before pulling the trigger (and the shortish KX250F chassis didn’t help calm it down). The 450 four-stroke, while no dog, didn’t create the same kind of hesitancy. With the advent of new technology and a revised approach to the engine characteristics of four-strokes, the smooth and rider-friendly four-stroke powerband trumped the 500cc engine (not in pure speed, but in usability).

In the concept department, the Service Honda KX500AF seems to be an ingenious idea. Melding the power of a 500cc two-stroke with the technology of a 250cc four-stroke sounds like a match made in heaven. The power-to-weight ratio is unheard of among motocross bikes, but this marriage isn’t perfect.

2009 KX500 AFThe Service Honda KX500AF is a bike that Kawasaki should have built and sold ten years ago. Unfortunately, during the interim four-strokes took over the motocross market and killed production 500cc two-strokes.

MXA test riders quickly discovered that the stock KX250F suspension needs help in order for the KX500AF to corner well and soak up bumps. We ran 90mm of sag in an attempt to keep the rear end up in the stroke, and we also turned in the high- and low-speed compression. These changes helped the front end stick better through corners, but eventually a stiffer shock spring was required to help resolve our issues. We also struggled with the front end; there was a push from center-out in corners. Not surprised by this feeling (the stock KX250F has the same sensation), we swapped out the 23.5mm offset triple clamps for a set of 22mm clamps. This change made a solid improvement in the turning ability of the Service Honda KX500AF. Finally, the short wheelbase of the KX250F frame had limitation when matched to the 500cc powerband.

On the plus side, every rider raved about the comfort of the KX500AF, especially seasoned 500cc riders. The KX500 of the olden days was targeted towards the offroad and desert crowd, meaning that the ergonomics were not necessarily cozy for a motocrosser. The switch to the KX250F chassis, which was purpose-built for motocross, was a massive ergonomic improvement over the old KX500 frame.

What surprised us most about our Service Honda KX500AF bike was that the rear wheel came with a small rim and 110/80-19 tire. The power of the 500cc engine begs to have a wider rim and larger tire footprint on the rear to maximize traction. Bike vibration was fairly evident, although most riders hardly noticed any vibration once they rode a few laps around the track.

VERDICT: WHAT DO WE THINK?

2009 KX500 AFThe short wheelbase of the KX250F chassis did not fare well with the powerful KX500 engine. 

The Service Honda KX500AF is a bike that Kawasaki should have built and sold ten years ago. Unfortunately, during the interim four-strokes took over the motocross market and killed production 500cc two-strokes. It’s a dream to think that the Service Honda KX500AF could revive the once-strong two-stroke. If you’re looking for the power of a 500cc two-stroke with the technology of an aluminum frame, the Service Honda KX500AF is the bike for you. The Service Honda KX500AF retailed for $10,999.728X90_1