200cc 4-Stroke MX Weapon

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200cc 4-Stroke MX Weapon

Postby 4Strokes » Fri May 27, 2016 4:07 pm

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200cc 4-Stroke MX Weapon by Matt Baldwin
No question, four-stroke mania is sweeping the off-road motorcycle world. Big-bore specialists are switching in herds, waiting lines for CRF450s are longer than our last president's list of impeachment articles and specialty companies are turning out exotic two-wheeled high-dollar museum-quality machinery as fast as they can off-load another boxcar of aluminum. So far, however, most of these efforts are concentrated in one of two areas - either open-class size bikes or the "Hangtown effect" - small-to-mid-sized motors in small frames, and not much in between. Obviously, motor limitations of small-bore four-strokes do not lend themselves effectively to full-size running gear. Herein lies the lack. But it doesn't have to be. Born of necessity and available parts, I give you the CR200X motorcycle.
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Imagine the situation: You're a 40+ Expert who has always preferred small-bore off-road bikes, even after successfully campaigning on CR250s for a number of years. You have an older model CR125 with a Pro Circuit motor and Race Tech suspension. You enjoy beating guys half your age on 125s and other 40 year olds on 250s and 500s, but it's getting a little tiring flying around on a two stroke 125. You also have a 1988 XR200R that's been patiently waiting in the crate for its master's retirement. 1988 was the last year for the XR200R with the lighting package, hotter cam and longer legs. I had high hopes for the little XR, but reality was quick to set in. Even with aftermarket motor and suspension investments, the package fell painfully short of even my meager capabilities. So what do you do? If you know a guy named Keith Leighty, the answer is easy.
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Keith is known throughout the southwest for his motorcycle building mastery. As a Honda dealer in the early 70's, Keith was figuring out the best way to put XR motors into CR frames before the first Elsinore in America even hit the track. Keith's Baja history is especially storied. Keith's four-stroke 125s in CR frames have won the Tecate 500 twice and have led the Baja 1000. In a year when NO factory or support Honda even finished the Baja 1000, Keith finished 3rd on a four-stroke 250 in a CR frame. And though they might not admit it, Team Honda itself has solicited Keith's expertise on more than one occasion. One year Keith built a bored and stroked XL175 in a CR125 frame that ran an honest 97 mph on the dry lakebeds. After shadowing Malcolm Smith's Husky 400 for about 50 miles, the king himself had to inquire at a pit stop - "What the heck is that thing?" Keith's tri-level shop, garage and private motorcycle museum are worthy of another story.
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The request was simple, build a competitive small-bore four-stroke that would look, feel and handle like a CR. I've seen conversions that graft an entire XR cradle onto a CR backbone, but Keith feels that too much structural integrity is compromised with this strategy and foot peg relocation can get messy, too. CR frame material is still superior as well. Keith chose to build a new cradle utilizing ONE PIECE of chrome moly tubing that was creatively bent to curve from foot peg to foot peg THROUGH the original CR down-tube. The real artwork is the one-piece aluminum combination rear motor mount/swingarm spacer. This let the XR countershaft end up only 1/4" forward of the original CR countershaft location. All the stock CR plastic is used and a handmade custom oil cooler slides through the frame and sits in place of the CR radiators. The XR wiring harness was recreated to allow all the electrics to reside in the airbox, and you can still plug in the stock headlight if you want to. A one-piece aluminum cone was crafted to join the stock CR airbox with K&N filter to the flat-slide Mikuni. The head is graced with a 3-angle valve job and Keith's proven port and polish work. Web Cam resurrected their awesome competition grind from the ATC 200X racing days and put it on my new core. Their double spring kit along with titanium keepers complete the top end package. Among the dozens of motors stashed in Keith's shop, he found that some ATC 200X motors had cylinders with finning substantially larger than the XR version. Since they're interchangeable (with some minor grinding), it made sense to include the big-fin barrel with a performance minded package. After plugging in an LA sleeve, we went up a couple bore sizes (with room to spare) and installed a 12:1 Wiseco piston kit. It's still a true 200 and legal for most 125 classes without compromising reliability or ridability like some other kitted motors that measure out to 220 or 240. Various exhaust pipe diameters and lengths were experimented with, and dyno testing showed that a 1-3/8" diameter pipe between 34 and 35 inches long provided the best power. Constructed from scratch, the narrow-profile pipe follows the exact path of the CR stinger and is tipped with an IDS SuperTrapp. Computer generated shroud and swingarm decals printed on commercial airline sticker stock (good up to 800 mph or so) and steel-braided oil cooler lines help complete the works factory look. The guys at Sun Honda in Denver and Lone Star Motorcycle Supply in El Paso were invaluable in helping collect parts and technology for the project.
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The final product? Factory fit, finish and feel, 214 pounds wet and an absolute gas to ride. Thanks to the pipe and the remounted shrouds, the bike is actually narrower than a CR125. What else? 3rd gear starts and 5th gear wheelies with two-stroke throttle response. Al Baker's XRs Only built a Showa cartridge fork just for this bike and the fat shaft custom-built Ohlins out back is equally awesome. Beware - if the track is rough or slippery or if your two-stroke 125 isn't running just right, you just might get beat to the next corner. And what's the point? With the right parts and the right connections, fairly exotic full-size small-bore four-stroke fun can be had without mortgaging your house or your kid's college education.
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Credits: Article written and submitted by Matt Baldwin. Publised and edited by 4Strokes.com.

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