TWO-STROKE TUESDAY: KX500AF ENGINE IN A KX250F CHASSIS

On May 17, 2017 Last updated Jan 23, 2018

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This is flashback test from eight years ago of a Service Honda KX500 engine in a KX250F chassis — obviously they have moved on from this pairing, but it is still interesting two-stroke reading and fun to think about.

Imagine, for a minute, the perfect motocross bike. Let your mind wander to unfathomable, unrealistic ideas. Draw up a sketch of the machine on paper, and don’t forget the vivid details. Odds are you would want to construct a motocross bike with a light, narrow, compact chassis. Cradled in the frame would be a potent engine with plentiful horsepower, yet one that was smooth and easy to ride. Now imagine that this dream can become a reality, because that’s what the MXA wrecking crew thought when we tested Service Honda’s CR500AF (AF stands for Aluminum Frame). With an old-school CR500 two-stroke engine cradled in a 2008 CRF250 aluminum chassis, test riders thought they crossed through the pearly gates and had gone to heaven.

MXA test riders are a fickle bunch. We want what we can’t have, and when we get what we want, we aren’t satisfied. Case in point, we yearned for a bike like the Service Honda CR500AF, because it had all of the attributes of our dream bike. On the short list were strong power, solid handling, decreased weight and affordable rebuilds. Truly, the Service Honda bike covered the best of both worlds. Best of all, it was an easy dream to see to fruition; in one short phone call our dream became a reality.

When we tested the Service Honda CR500AF, we enjoyed the raw power of the engine, coupled with the lightened handling and feel of the CRF250 chassis. However, having plentiful experience with 500cc two-strokes from before they were put out to pasture, we knew that the Kawasaki KX500 engine had some significant benefits over the CR500 powerplant. Amazingly, Service Honda was on the same page. They felt that the broader powerband of the KX500 engine in a modern chassis might make the dream even more dreamy.

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.

Enter the KX500AF. Service Honda was selling the Kawasaki version of their highly popular 500cc conversions in a KX450F chassis. After much deliberation, A.J. Wagonner decided that for 2009 the revised KX250F chassis and lighter frame would be better suited for the KX500 engine. How light is the overall package of the KX500AF? It is roughly six pounds heavier than a stock KX250F and about 15 pounds lighter than a stock KX450F.

SHOP TALK: WHAT’S IT ALL ABOUT?

How light is the overall package of the KX500AF? It is roughly six pounds heavier than a stock KX250F and about 15 pounds lighter than a stock KX450F.

Our most pertinent questions for Service Honda centered around the amount of time it took the technicians to build the KX500AF. After all, parts off two dissimilar bikes needed to be melded together in order to complete the project. According to A.J., 40 hours were spent from concept to completion. Most of the time (roughly 38 hours) was devoted to the chassis, while only a small fraction of the total time (about two hours) was spent building the engine. While Service Honda can build a KX500AF relatively quickly, there is still a 30-day waiting period (and initial $1000 deposit) upon ordering. Service Honda builds five to ten bikes at a time to speed up the production cycle.

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.

TEST RIDE: TRYING TO TAME THE BEAST

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).

The MXA wrecking crew has included a number of seasoned 500cc two-stroke riders (and we tested with former 500 National Champion Chuck Sun, the late 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.

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.

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.

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. The KX250F frame wasn’t the best match for a 500cc two-stroke. The MXa test riders were not surprised by the quirky handling (the stock KX250F was odd with its designated engine).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 serious 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 DID WE THINK?

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

The Service Honda KX500AF was a bike that Kawasaki should have thought about at the time. Unfortunately, during the interim four-strokes took over the motocross market and Kawasaki has eschewed two-strokes. It’s a dream to think that the Service Honda KX500AF could have revived the once-strong KX500 two-stroke engine—but that feat wasn’t accomplished until years later when Sean Collier raced his KX500 to glory. The retail price when we tested this bike was $10,999.

NEWS UPDATE: SERVICE HONDA TO STOP BUILDING THE CR500AF?

For more info go to www.servicehonda.com/page/af-motorcycle