Are you a believer in light, easy-to-maintain two-strokes but are tired of the huge displacement and torque advantage that four-strokes enjoy? Boyesen Engineering may have just what you're looking for. We tried two versions of its 285 big-bore kits to see how they compared to the now-dominant thumpers. Boyesen modified a YZ250 and an RM250 with its 285 big-bore kits as well as a slew of bolt-on modifications including a Rad Valve intake, Powerwing carburetor insert and Supercooler water pump kit on both bikes. The crew also installed their Shokout left-side grip, which provides a noticeable reduction in the wear and tear on your left hand and wrist. Both bikes were immaculately prepared, and we couldn't wait to spin laps on the Budds Creek motocross track the day after the 2007 Motocross of Nations.
To get a mix of opinions, we gathered three very different testers: our resident doctor Gardner Tarlow, a solid Vet Novice rider (5 feet 10 inches and 190 pounds), his buddy Evan Benehoff, a rather large Vet Intermediate rider from Lansing, Michigan (6 feet and 240 pounds) plus myself, another Vet rider but on the smaller, lighter side (5 feet 10 inches and 165 pounds) of the rider spectrum. We were all very impressed with the performance of both bikes as they were jetted perfectly, had good bottom-end power and were very predictable, which made it easy to ride the bikes hard right from the beginning. The Powerwing and Rad Valve likely contributed to the nice snap available from the first crack of the throttle. The Yamaha had a little stronger bottom-end, and both bikes revved well without any sign-off common to big-bore kits. Modern two-strokes have an almost four-stroke feel to them now, with smooth, controllable power throughout the rpm range, and the Boyesen big-bores just magnify that. They will still light up the tire more easily than a thumper, which takes some getting used to, but the bikes have the benefit of a much lighter feel that helps in changing direction and jumping. Make no mistake-most riders will still find a four-stroke easier to race when a track is hard and slippery, but these Boyesen-modified big-bores narrow the performance gap and provide a legitimate alternative with benefits such as lower cost and easier maintenance. Eyvind Boyesen was one of the pioneers of two-stroke performance in the 1970s and held many patents that were licensed by most of the major OEMs, so it should be no surprise that his company continues to produce some of the finest performance parts available. So if you want to pump up the performance on your trusty 250 two-stroke, contact Boyesen Engineering.
Parts
Boyesen Engineering:
www.boyesen.com
800.441.1177
285 big-bore kit: $1200
Rad Valve intake: $179.95
Shokout grip: $89.85
Powerwing carb insert: $129.95
Clutch cover: $92.95
Ignition cover: $82.95
Supercooler water pump kit: $189.95