Roost Boost
ROOST BOOST
More muscle for Yamaha's WR25OF
MARK HOYER
CW PROJECT
DESPITE WHAT YOU’VE HEARD, THERE ARE SOME REPLACEments for displacement. More revs and hotter cams, for example. Porting, for another. Losing weight (your own) is oft overlooked.. .but hey, get out the boring bar, no one should have to give up a decent meal!
Taking that food lover’s approach, we set out to build a bigger, badder Yamaha WR250F, and the Cycle World Project WR315F was bom. Yes, after a thorough test flogging, our 2003 WR had shown its fun nature on every kind of terrain, but we couldn’t help asking ourselves what the bike would be like with a pumped-up bottom-end boom and more top-end zoom.
Rick Peterson at RPMs had the answer. His West Covina-based company is an empire of boring bars and stroker cranks. Peterson has been attacking twoand fourstroke engines with such equipment for some 20
years, recently
getting into the business of chipping large-capacity dirtbike cylinders out of billet aluminum, such as the one fitted to our tester that bumped displacement to 315cc.
Called the Ice Cube, the new cylinder is CNC milled out of a big block, fitted with a forged alloy sleeve that is plated in-house with nickel silicon carbide (like Nikasil) for long life. At displacement jumps this large, the new-cylinder method has the advantage of increasing water capacity for better cooling and eliminating the spacer plate that would be required if using the stock barrel with the longer stroke. Further, in place of a standard head gasket is a Viton O-ring, which Peterson says is much more reliable.
Inside, an 81mm Wiseco Pro-Lite three-ring piston (designed by Peterson) whips up and down in a 61mm stroke (stock dimensions are 77 x 53.6mm). A dished piston top is used because Peterson feels it provides a combustion chamber more resistant to detonation than the flatter-top, higher-compression stock Yamaha piston, therefore allowing the use of 91-octane fuel, even in the larger bore.
An 8mm longer rod is fitted, using an I-beam design for increased strength. The cylinder head is left stock, but the airbox is amply vented to provide the increased flow required by the bigger lung.
Like most engine builders, Peterson will tailor power and engine spec to your taste, but we ran the WR cam for a mellower, more tractable powerband. You can opt for the YZ-spec full fire-breather but, honestly, we wanted to build a good trailbike.
The kit is pricy, running $3050 for all the stuff you need to do the work yourself, or send your motor to RPMs and for $3195 they’ll put it all together for you. Either way, RPMs provides two piston kits and two gasket sets to encourage proper maintenance.
As for tuning, the infamous “gray wire” on the Yamaha’s ignition box is connected, while the throttle-position sensor is disconnected-with the big-bore, Peterson feels the TPS dials in too much advance at low revs, making throttle response abrupt. Without information from the sensor, the ignition reverts to a single default map that changes advance based on rpm alone.
In the excellent Yamaha WR250F Owner’s Service Manual, it says that if you encounter “hard breathing” and “lack of speed” between V4and 3/4-throttle openings you should lower the carburetor jet-needle clip position-to which we might add maybe you should try also lowering your fork position at meals a few notches, and perhaps add some sit-ups.
Seriously, the jetting section was an important part of the manual because we definitely needed to have the carb tuned to suit the larger lung. JD Jetting provided a kit for the WR250F that worked exceedingly well. Jetting for a 250 Stocker that works in a modded 315? Yep. Contrary to what you might expect about a bigger engine needing more fuel, JD man James Dean (an aerospace engineer in his “day” job) finds that some big-bore bikes actually run better with leaner jetting due to higher-velocity, less-dense intake air and increased vacuum from the bigger cylinder. Ran like a champ, so we defer to the engineer on this one.
The Pro Circuit exhaust features a titanium header and Type 496 silencer. As it says on the side of the can, it’s a “4Stroke System Developed for Reduced Noise Output,” and is pleasantly quiet but with good sound. Being USFS-approved, it features a spark arrestor. It also removed 3.3 pounds from up high, further enhancing the WR’s already light feel.
Gearing was left stock (13/52) due to our penchant for riding first-gear rock trails from hell, but it would easily pull 14/47 with its newfound muscle.
So we didn’t look like goons riding the bike at moto practice days (oh, the holeshot...), we left off handguards. To make sure levers survived the inevitable cartwheeling (yep, twice) we fitted ARC levers. These trick pieces are machined from 6061-T6 aluminum and feature a springloaded “reverse” pivot so they fold out in crashes to further protect from breakage. Plus, they just feel good to use because the material and design make them stout and substantial under finger. They ain’t cheap, and it was necessary to trim the rubber boot on the brake lever so it didn’t interfere with the reach-adjustment bolt.
Renthal 971 aluminum handlebars replace the steel Stockers (which often bend in as little as a stiff breeze). In addition to being strong, the bars are taller and wider than stock, giving a much more roomy cockpit and easing stand-up riding. For 2005, WRs come stock with Renthals, so take that off the list if you want to build a 315 starting with a brandnew bike this year.
With little to complain about from the suspension it was left stock, although Fineline serviced both ends and replaced the pivot bearings in the rear linkage. We considered changing the rear spring, but found that the stocker could accommodate even our big-guy testers and maintain recommended sag.
We definitely blew the budget on this beauty, as you can see, and used so many trick add-ons it possibly negated any weight-savings brought about by use of the Ti exhaust. Looks bitchin’, though. A Ceet Gripper seat cover adds to the factory flair.
Okay, a little over the top for the casual trail rider (the stickers definitely are), but the 315 is actually an awesome combination of qualities, particularly useful if you’re not quite as fit as you used to be. It retains the light feeling and quick, easy handling of a 250cc four-stroke, but with significantly increased bottom-end snap. Wheelies are much easier and gear selection isn’t nearly as critical. In the hands of a fit expert, you’d be hard-pressed to find a bike that could attack a tight trail as effectively while still having the lungs to keep singing in wide-open spaces.
If there are any complaints, it is that throttle response is very lively, borderline too snappy for lower-traction conditions. At the end of a long, challenging day, it becomes tough to manage the power in slippery stuff. This, even in the face of choosing a more mellow engine spec.
Overall, though, the new engine personality is dramatically different than the WR we used to know. The motor now pulls really strongly until the upper reaches of the rev range, where power flattens out a bit. You’re probably dealing with two things here: One, that there is so much more power on the bottom, top-end power just doesn’t shine as much; and two, the stock porting is optimized for a 250cc engine’s air requirements up to the 13,500-rpm rev limit, and the extra inhalation of the 315 is a bit much at the same high revs. But now you don’t need to rev it like a 250, because it isn’t one. The other part of this is that we chose a more enduro-like powerband. There may be more peak power locked inside, but in the interest of building a fun playbike with power mere mortals could harness, we chose the trail-friendly option. Overall, the WR315F provides a superb balance of power and agility and makes one of the most fun trailbikes about 30 percent more so, There are many reasons to consider such an upgrade on your WR, particularly if you find yourself asking your buddies, “Does this bike make me look fat?” If so, consider a WR315F. And treat yourself to a nice meal.
SUPPLIERS
ARC Levers 1428 E. Borchard Ave. Santa Ana, CA 92705 714/543-0362 www.arclevers.com Clutch/brake levers $70 ea. Ceet Racing 1220 Keystone Wy. Vista, CA 92081 760/597-1077 Ceet Gripper seat cover $70 Fineline Suspension 7522 Slater Ave. Huntington Beach, CA 92647 714/842-9253 www.finelinesuspension.com Fork service $150 Shock service $150 JD Jetting P.O. Box 6044 Federal Way, WA 98063 253/939-7114 www.jdjetting.com Jet kit $70
Pivot Works 3025 Justin Dr. Urbandale, IA 50322 515/402-8000 www.pivotworks.com Shock linkage bearing kit $80 Pro Circuit Racing 2771 Wardlow Road Corona, CA 92882 909/738-8050 www.procircuit.com Ti headpipe $275 Type 496 silencer $450 Renthal America 24950 Anza Dr. Valencia, CA 91355 877/736-8425 www.renthalamerica.com 971 Aluminum Handlebar $90 Rick Peterson Motorsports 1011 W. San Bernardino Rd. Covina, CA 91722 626/967-3052 www.rpmsbigbore.com Ice Cube big-bore & stroker kit $3195
