Motorcycle test riders are a strange lot. The best are able to separate their personal peccadilloes from their professional responsibilities. Nowhere is this more evident than in the bikes that MXA test riders choose to race–when they are just racing for themselves. Take this year’s crop of 250cc four-strokes. Every MXA test rider voted that the Kawasaki KX250F was the best bike. Their logic was simple-the KX250F hit the hardest, pulled the farthest and was the fastest 250F of the bunch. In a class where horsepower is at a premium, that was enough to hand the Bike of the Year prize to the KX250F. Yet on a weekly basis the same test riders who voted unanimously for the KX-F can be found racing Suzuki RM-Z250s at Glen Helen, Gorman, Piru and Competitive Edge. How can this be? After all, the RM-Z250 didn’t win MXA’s 2007 250F shootout because of its overly soft suspension, tall gearing, questionable clutch, glitchy neutral and lack of top-end power. But, and this is a big but, the RM-Z250 is the best handling motocross bike made. Hands down! And as any test rider will tell you, “It’s easier to make a slow bike fast than to make a bad handling chassis handle.”
The fact that the MXA wrecking loves the 2007 Suzuki RM-Z250 is no secret. But to get full benefit out of the fabulous chassis, we had to solve the RM-Z’s nagging issues. And that is exactly what we did with our Yoshimura R&D-modified RM-Z250.
SHOP TALK: WHAT IS IT ALL ABOUT?
The 2007 Suzuki RM-Z250 is a brilliant bike, blessed with the most accurate chassis ever to grace a 250 four-stroke. It is also seriously flawed. Our goal was to eliminate the flaws with the judicious application cubic dollars. Be forewarned we went all out, spent money like it was water, and went so far as to fix things that weren’t really broken. It’s up to you to pick and choose the mods that make the most sense for your racing style (and bank account).
In stock trim the RM-Z250 needs two things: (1) It needs more topend. Although the bottom-to-mid is luscious, the stock engine signs off on top and hangs. (2) It needs stiffer suspension. The forks and shock have to be resprung and revalved to keep them higher in the stroke and to lessen bottoming.
Engine mods: We didn’t see any reason to pussyfoot around with the engine. We went straight to Yoshimura R&D. Although Yosh is best known for their exhaust systems, they have a complete engine shop (one that cut its teeth on AMA Superbike engines). Yoshimura offers three levels of engines mods. Stage 1 includes cylinder head porting, valve seat refacing and resurfacing of the cylinder head. Stage 2 includes everything in Stage 1 plus a high-compression CP piston, Yoshimura ST-R camshaft, valve spring kit and new gaskets, guides and seals. Stage 3 includes everything in Stage 2 plus bottom-end blueprinting.
The MXA gang opted for Stage 2. It had the performance parts that would make the biggest difference. The retail for our Stage 2 kit was $1725. But that was just the beginning. We added a Yoshimura RS-2 Pro99 Carbon exhaust system ($895), CV4 silicone radiator hoses ($160), complete Hinson clutch ($240 basket, $225 pressure plate and $480 inner hub) and Works Connection engine plugs, Elite clutch perch and billet throttle tube.
Suspension mods: Unless you weigh under 150 pounds, the stock suspension on the RM-Z250 is going to be too soft (and too undersprung). Faster, heavier or serious riders will need to go stiffer. Our quick solution was to send the whole Showa kit-and-kaboodle to Factory Connection to have them revalve the forks and shock.
Factory Connection’s fork mods, including revalving, works oil lock collar, low-speed valves and pressure springs, ran $335. Factory Connection kept the stock 0.44 fork spring rate. Factory Connection’s goal was to increase low-speed compression damping to keep the forks higher in the stroke. Their first attempt left us cold. It didn’t stay high enough in its initial stroke and was harsh in the midstroke. We sent it back to be revalved. The second setting was a big improvement.
The Factory Connection shock mods cost a little over $200 and included a revalve, ultralight 2.5 weight oil and a new high-speed compression adjuster. Again, Factory Connection stuck with the stock 5.4 kg/mm spring, which saves the consumer money, and tried to increase damping pressure enough to compensate for the RM-Z’s overly soft feel. The shock was well balanced and had a very controlled feel.
Miscellaneous mods: For personal taste we added theses parts: Fastway’s ultra-wide footpegs ($130); Flu Design graphics ($99.99) and seat cover ($30); Dunlop D756 race Replica tires; Works Connection skid plate ($70) and case guards ($55); Uni Filter air filter ($27); Applied top triple clamp ($395); Renthal Twinwall 997 bars ($120), Kevlar grips ($20) and 49-tooth rear sprocket ($65); Faast rim lock spacers ($13) and rear brake clevis ($54).
TEST RIDE: HANG ON AND PRAY
What is the difference between a stock RM-Z250 and Yoshimura’s ported, piped and pistoned RM-Z250? Night and day. The Yoshimura engine was fast. Best of all, it didn’t give up any of the RM-Z250’s fabled low-to-mid, it just added a healthy dose of overrev on top. Every test rider loved the way the RM-Z ran with the Yosh Stage 2 kit. It was perfectly jetted, picked up cleanly off idle and screamed into the top without hanging like the stocker. The added dimension of more rev produced a bike that could be used as a torque monster or as a rev ranger. It was sweet.
When it came to the suspension settings, no MXA test rider could live with the stock stuff. It squirmed under the weight of a fast rider and bottomed in the big stuff. Factory Connection’s fork and shock mods stabilized both ends of the RM-Z, keeping it out of the danger zone and giving the rider a more stable platform to work from. Although most MXA test riders would have preferred stiffer spring rates, the Factory Connection setup was ballpark for riders under 175 pounds.
VERDICT: WHAT DO WE THINK?
When you love a bike in spite of its flaws, that is true affection. Fixing the powerband and the suspension only made our love grow stronger. We heartily recommend Yoshimura R&D’s Stage 2 engine kit. It romped. It turned the mild-mannered RM-Z250 into a multipurpose motocross weapon.
For more information contact Yoshimura R&D at (800) 634-9166, Factory Connection at (800) 221-7560, Hinson at 909-946-2942, Works Connection at (800) 895-8290, Fastway at (208) 466-4762 and Renthal at (661) 310-0060.