This past season saw Suzuki release its first fuel injected quarter-liter motocross bike, one which garnered substantial acclaim even though it hit the market slightly later than the competition. It’s for this reason we selected the yellow 250cc Japanese four-stroke as the basis for an on-going motocross project bike with the help of our friends at Yoshimura.
Yoshimura Research and Design went bonsai on Suzuki’s 2011 RM-Z250 netting a motocross bike that was not only faster but easier to ride.
It’s a well-know fact that the boys at Yosh have their hands in with factory Suzuki dirt bikes and Suzuki ATVs, but what does that mean for the regular consumer? They can build fast bikes, but more importantly, can they make it rideable for variety of skill levels? Throw loose dirt and big jumps into the equation and there’s far more than outright power that equates to fast motocross lap times. We dropped the bike of at their Chino, California-based shop and a month later it was time to find out.
Starting from the inside out, the boys stripped our RM-Z to the frame and removed the engine. From here their Stage 3 process began. The top-end came off, cylinder went one way and the cases were split and sent the other direction.
The RM-Z250 engine features Yosh’s Stage 3 engine kit designed for motocross racing. Not only does the set-up crank out more power it is reliable too.
Why go to such lengths on what is an already very capable and strong engine? When adding large horsepower gains, a key ingredient is a bottom-end that can handle additional thrust – the crank and transmission especially, as both are highly stressed. This involves sending the gears and spline through what Yosh calls its ISF process (Isotropic Superfinishing Process). It tumbles the pieces through very fine grains of sand, almost like minute sand blasters, to ensure a smooth and strong finish. They are then under-cut for better engagement when shifting between gears. This is all done by Yosh in-house. This is also where they blueprint your engine’s gear sets just as is done on a Formula 1 machine, allowing your engine run more freely through decreased friction and proper interaction of reciprocating parts. Keeping the internal parts lubricated are Maxima’s latest oils.
CP Pistons supplied the lightened and reshaped piston. Bore remains the same but a new dome shape and different head gasket set raises the compression to 14:1. The cylinder head is then ported to Yoshimura’s own specifications while cams are sourced from their aftermarket catalog. Intake lift is set to 9.0mm with 240-degree duration, while on the exhaust side lift is 8.5mm and duration is the same. Remember, these are numbers tuned specifically for our engine, so we don’t recommend setting yours the same unless done by a professional on a machine with equivalent engine work.
(Above) A Hinson clutch internals provide consistent clutch performance whether you’re on the first lap or 30-plus minutes into a moto. (Below) Ultra-trick magnesium TCR hubs are fitted to black Excel wheels with a Renthal chain spinning the drive sprocket.
Pulling in fresh air is a Twin-Air filter, while expelling spent gasses is Yoshimura’s exquisite RS-4 full titanium exhaust with carbon muffler can. The system is beautifully made and fit extremely well. Suzuki’s well-rounded powerplant remains solid throughout the rev-range with added low-end grunt and additional over-rev. We found that it helped the Suzuki pull multiple gears in different situations. This is further aided by Yoshimura’s PIM-2 Injection Box, giving one the ability to tune the power curve on the dyno. The Suzuki was originally setup to run with VP’s U4.4 race fuel for the first test, then re-tuned for pump gas later as running VP all the time gets expensive. Yosh’s own data box was fitted as well to log the engine’s running conditions.
Several Yosh-built hard parts were fitted throughout, including a TDC (Top Dead Center) inspection plug, timing inspection plug, axle adjustment blocks, brake clevis and hot start nut. A full Hinson clutch setup – basket, hub, pressure plate, spring set and case cover – are beefier than stock to handle the added ponies; water hoses are made by CV4 and designed to handle high-heat.
Rental provides the handlebars and grips as well as the final drive sprockets, while an RK chain and rims are mated to TCR hubs. Factory Connection re-valved and re-sprung the suspension for a 145-pound tester, while Braking wave rotors, XTRIG triple clamps and Dunlop Geomax MX51 tires bring the handling on par with the monster engine. Rounding out the uber-trick package are Lime Nine custom graphics and an SDG seat cover that went on to aid in rider comfort and added grip.
Motocross bikes these days come from the factory in near ready-to-race form. Stick on a set of numbers and hit the track. It’s for this reason that making massive strides in performance isn’t always the easiest, as they are so well engineered and full of technology from the factory. That isn’t to say it’s impossible.
Because the bikes are setup for a wide range of rider sizes, skill levels and terrain, there are some comprises that have to be made. This is why one of the quickest and easiest ways to gain a performance edge is with suspension. For our project bike we used Factory Connection, who factors in rider weight, ability, type of tracks, etc, to come up with a spring, valving and oil-level combination.
We were blown away by the Suzuki’s set-up. The little RM-Z is extremely stable, yet steers with ease, while being compliant and supple over the small bumps. It’s so forgiving on the small chop that it feels like soaking up big jumps could easily induce bottoming. But that’s where the beauty of the Factory Connection set-up lies, as the progressive rate of the damping and springs renders the bike difficult to bottom over even the biggest jump. No out of control bucking through the whoops or kicking sideways when over- or under-jumping an obstacle – just complete confidence-inspiring action at all times. Stock suspension has come a long way in the past five years, but the aftermarket companies have still found a way to make it better, something quite impressive in itself.
“I was amazed at how well the Suzuki behaved through the chop and ruts,” says pro test rider Matt Armstrong. “It’s been awhile since I’ve spent some time on a proper race bike and it was quite eye opening; simply aim, shoot and the bikes goes wherever you want it, whenever you want it.”
And it’s a darn good thing the suspension and chassis of the RM-Z is so user-friendly, as putting the added power Yosh has pulled from the yellow bike to the ground is no easy task. Though the high-compression piston makes the bike hard to start when cold (it starts on the first kick once warmed up), once up and running the power is downright impressive. Just twist the throttle in neutral and one can tell this bike is special, as the snap and speed which it builds revs is almost scary. Some bikes have bark but don’t back it up with bite, but if anything the Yosh 250 is the other way around.
The range over which the power is produced is equally as astonishing. The RM-Z starts building down low and pulls through the rev range with venomous fury. This leads to an equally impressive over-rev that seemingly never ends. So much so that at times it feels as if the engine is straining, almost like a grenade with the pin pulled. Except it never blows, just forever accelerating until only slightly tapering off at extremely high revs.
The Yoshimura DATA system allows the rider or tuner to adjust the engine fuel and ignition settings trackside based on rider preference or terrain conditions.
“The motor on the bike is one of the strongest 250Fs I’ve even ridden,” adds Armstrong. “It’s unreal what Yoshimura has been able to pull from that small little engine. It snaps to life with a howl, pulls off the bottom like a freight train but has no adverse effect in top-end performance or over-rev, as one can hold a gear in several spots around the track that wouldn’t be even close to possible on a stock machine.”
Part of the reason the bike runs so well is the Yoshimura DATA system. The box works in conjunction with the PIM-2 ignition mapping system and self-tunes the motorcycle for the conditions at hand. Both the PIM and DATA box are plug-and-play ad can be easily installed, though the air-fuel sensor from the DATA system may require some exhausts to have a small hole drilled in them. This sets fuel adjustments at each throttle position and rpm range, and is done through the addition of an air-fuel sensor and amplifier box, allowing the bike to be tuned without the use of a dyno or specialty equipment.
“One of the reasons the motor impresses me so much, on top of having great power and over-rev, is the usability and broadness of the power distribution,” Armstrong continues. “I’ve ridden several factory race bikes for tests and such, most of which had light-switch-like power. But the Yoshimura RM-Z could be ridden by my granny it’s so docile and behaves so well low in the revs, yet pulls hard as the speeds climb. It’s almost deceiving at first, not really feeling too outright quick when putting around and scoping the track, but give it some stick and she really comes to life. What a great combination.”
While the mods aren’t if you’re looking for the ultimate RM-Z250 racebike Yoshimura can get you dialed.
Armstrong summed up our Yoshimura project bike nearly perfectly, as the truly mesmerizing feature of this motorcycle isn’t one particular thing, it’s a combination of all of them, which when tuned by the likes of Yosh equate to one of the best handling, fastest 250F motocross bikes we’ve ever swung a leg over. Not to mention also being “granny friendly.” It may cost a pretty penny, but being competitive at the highest levels of the 250F class comes with a price. Yoshimura obviously knows how to make the most of those hard-earned dollars. There’s definitely a reason they work with top race teams to build championship-winning bikes.