How do you fix a shootout-winning bike? This was the question I was faced with in the weeks following our 250F motocross comparison (March 2011 issue). I’d ended up with the Kawasaki KX250Fa machine that our picky test riders had dubbed the best bike in its classand my initial thought was to just play with the clickers and leave the thing mainly stock. However, after combing through our shootout notes and riding the Kawi on several more tracks, I came up with a short yet definitive list of mods and changes for the green machine. In order to maintain touch with the realities of the workingman’s budget, I made an effort to fix the Kawasaki as organically as possible; I wasn’t going to just ship out the engine and suspension and wait to see what was sent back. Rather, I walked the project along every step of the way and was thus able to retain control and direct the nature of the bike.Engine
My main objective in modifying the Kawasaki’s powerplant was twofold: Massage the delivery for more meat upstairs and quiet down that infernal din that is the stock muffler. The first point was achieved through the simple application of Kawasaki’s Digital Fuel Injection calibration kit and one of seven preprogrammed aftermarket settings; in this case, the soft terrain map. With a leaner fuel setting and an advanced ignition map, this was our favorite of several settings that we tried as it gave the KX-F even more rage at higher revs. The power just didn’t sign out like it used to, and best of all we barely lost a thing in lower ranges, with the bike’s signature hard low-end scream remaining intact. Ah, the joys of tuning in the computer age!
The sound issue was just as easy to remedy: We simply swapped out the stock exhaust for a Leo Vince Full-Titanium system, and then inserted the supplied spark arrestor. In combination with the DFI mods, this setup was an even stronger, smoother version of the stock race bikelike feel, and it sounded great. We had an excellent experience with a Leo Vince system on this machine in the past and expect that it will hold up for an entire season and then some. Mission accomplished!Suspension And Handling
The goal of fine-tuning this 250F’s suspension was to correct the slightly undersprung feeling in the fork. As you remember, Kawasaki graced the 2011 version of this machine with its new SFF (Separate Function Fork) technology, and from the very beginning we felt as though the fork was soft, especially for faster and heavier riders.The solution to this was found during a day of testing with Kawasaki’s technicians, where we started off at our best stock setting of five clicks out on compression, with stock preload of 5.5mm on the stock (9.1N/mm) spring. After establishing a baseline feel, we added 3mm (12 clicks) of fork preload and found an improvement in corner handling, where we could charge into turns a bit harder without the fork settling too much. This setting made the front end deflect at higher speeds, though, so it was back to the drawing board.
We then tried a 9.5N/mm (stiffer) fork spring (remember, there’s only one spring), and although it felt tall in the front end and slightly unbalanced we knew we’d found something magical on hard hits. Our next move was to move the fork up in the clamp (from the stock 7mm height to 10mm), as well as to tighten down the steering head for less looseness at higher speed. Boom! This was the setting we’d been looking for, as it allowed for better high- and low-speed damping in the fork on hard hits, as well as more stability when entering turns and at higher speeds. I wonder if Jimmy Lewis would notice if I took the Scotts steering damper off the KX450F?Miscellaneous Mods
With the DFI and fork fixes in place on the KX250F, I knew that anything else I did would just be a bonus (as well as a hit to the ol’ budget). In light of this, I kept it simple. Since we’d whined about the quickly hammered look of the stock machine, I took the time to rattle-can the beat-looking engine covers, and I replaced the shredded stock graphics with a full custom kit from Vention. Next, I equipped the Kawi with several T.M. Designworks components to replace the weak-link stock parts. Long haul testing of the KX250F in the past has proven how quickly the chain guide and guard can blow out, and I figured I’d do the bike a favor now and fix the issue before it became a problem. Finally, I swapped out the Dunlop MX71 hard-terrain tires from our shootout for a set of MX51 rubber, which I feel gave the bike better control on hard-to-soft-terrain transitions.Aftermath
In just a few short steps, a handful of smart revisions took the KX250F from great to amazing, and the overall feel of the bike was improved by performance that usually comes with a much, much higher price tag. In looking at the finished product, I’m extremely happy with the settings and decisions that we arrived at, and I can’t help but wonder if we just built the first 2012 KX250F. From here, the Kawasaki will join our Long Haul fleet and undergo additional testing, tweaks and routine maintenance as we continue to assess the shootout-winning machine.
|Leo Vince: www.leovince.com|
|T.M. Designworks: www.tmdesignworks.com|