ENTER THE DRagon
Pro Circuit Breathes Life
into our Suzuki RM-Z250
// BY THE TESTING STAFF OF TWMX
PhotoSà˜ DONN MAEDA
Back in the January issue, the Suzuki RM-Z250 and Kawasaki KX250F finished third overall in our 2005 250cc Four-Stroke Shootout, largely due to the fact that their engines-though improved for ’05-were not up to par with those of the Honda CRF250R and Yamaha YZ250F. With a boatload of low-end throttle response and good mid-range power, all that the great-handling yellow and green bikes really needed was some top-end overrev.
That being said, we knew that there was one place to turn to for a slam-dunk in the motor department. With more success building high-performance KXF and RM-Z motors than any other hop-up shop, we knew that Pro Circuit was the perfect place to visit for more horsepower.
We asked Pro Circuit to perform its most popular RM-Z/KXF motor mods to our test bike, and this is what they entailed… The cylinder head was stripped, cleaned and ported, and a Serdi valve job was topped off with new valve seals and a Pro Circuit valve spring kit, which also included keepers and spring retainers. The spring kit allows for the use of more aggressive cam profiles without the loss of valve control, especially at high RPMs. A Pro Circuit cam set, Pro Circuit clutch springs and a high-compression Pro Circuit piston round out the performance bolt-ons inside the motor. For flash and flair, a Pro Circuit Engine Plug Kit was installed. Anodized red, the billet aluminum plugs are for the chi-chi effect only. Finally, a Pro Circuit Ti-4 GP Low Exhaust System was bolted on. In addition to saving weight, the pipe’s lower mounting style also lowers the machine’s overall center of gravity.
We’ll cut to the chase right from the start and say that this is the most impressed we’ve ever been with a 250cc four-stroke project bike. And yes, that includes all of the Honda CRF250Rs and Yamaha YZ250Fs we tested last year; this RM-Z250 absolutely rips!
Right from the moment we fired up the machine, it was easy to tell that it was far from being the mild-mannered bike it used to be in stock trim. From the throaty exhaust note (albeit on the loud side) to the instantaneous throttle response and clean-revving nature of the modified powerplant, it was clear even in the pits that the bike was going to be a monster.
Once on the track, one question began to run through our heads, over and over again. “Why would any top pro race a two-stroke against one of these?” At the amateur level talent surely plays more of a deciding factor than machinery, but at the upper echelons of racing, electing to face off against a bike of this caliber with a pipey two-stroke is a ridiculous proposition!
Power down low feels just as good as stock, but the motor revs faster and pulls through the powerband more quickly and aggressively. On the dyno, we discovered that the modified motor actually has a little bit less way down low, but the manner in which it gains rpm on the track makes the dyno chart’s low-end readings almost impossible to believe. Getting over a seat-bounce double right out of a corner is easier on the modified machine (yes, we rode it back-to-back with our stock KX250F), and the bike powers out of tight, slippery corners with more “oomph” as well. Once the rpm begin to pick up, however, this is where the beefed-up motor really begins to shine. Where the stock bike tapers off and falls flat on its face, the PC motor continues to pull like it was a two-stroke. In sections that required us to grab the next gear on the stock machine, we found that the PC bike was able to rev out much, much farther in an efficient manner.
Side-to-side starts with the PC RM-Z and the stock KXF were ridiculous. The RM-Z would yank the green stocker right out of the gate and continue to pull away down the entire length of the start straight.
Did the motor mods have any adverse effects? As far as we can tell, the only undesirable side effect is that the bike is a little tougher to start. Even when cold, the modified motor likes the hot start, and we’ve not used the choke since we got the bike back from Pro Circuit. Thanks to the higher compression of the modified motor, it takes a manly stab at the kickstarter to get the motor fired up. One accessory that PC left off that we will surely order up is its billet aluminum kickstarter. Having used one last year, we know that the PC kickstarter has a much better leverage ratio than stock and makes firing the bike up a much easier task. At press time, we had logged nearly 10 hours on the motor and it has shown no signs of slowing down or wearing out. Pro Circuit recommends checking and/or replacing the valve springs every 15 hours, whether your machine is stock or modified.
With a price tag of $2,765.65 for the functional modifications, obtaining a motor of this caliber is not cheap by any means. Also of considerable added expense is the upkeep of the motor once it is built, though even a stock machine should be rebuilt with regularity. That, however, is the price of being competitive on a small-bore four-stroke. Four-strokes make more power than their two-stroke counterparts, but they cost more to maintain, and much more to rebuild if left neglected to the point that they blow up. If you are a serious racer, however, being able to grab a holeshot and remain competitive in the fastest sections of the track is all worth it.
Skill Level: Vet Intermediate
Years riding: 25
Height/Weight: 5’11″/180 lbs.
Bikes Recently Raced/Ridden: 2003 Yamaha YZ250F
I have ridden the stock Suzuki RM-Z250 and I wasn’t very impressed. After riding the Pro Circuit RM-Z250, however, I have a completely different opinion about the bike. This is the best bike I have ever ridden! The track I tested at is a Supercross-style track with lots of jumps and one thing I noticed was how well the power would roll on, making it very easy to clear any jumps out of the turns. You would think with such good roll-on that it would flatten out on top, but the motor keeps pulling and pulling. Having this motor would be a serious advantage on the racetrack. Like I said, this was the best bike I’ve ever ridden and I still have a smile on my face. – JS
Skill Level: Intermediate
Years riding: 6
Height/Weight: 6’2″/165 lbs.
Bikes Recently Raced/Ridden: 2005 Honda CR250R
Even though I race Hondas, it was easy to get used to the Suzuki. The bike was very fast up the big hills and pulled strong through the soft mud. The pipe was pretty loud, but I guess that’s what gives it the snappy bottom-end throttle response. The mid-range power was so strong, I could ride a gear tall in some long sweepers and build a lot of momentum. On top, the motor revved out very well and would not lose power before you grabbed the next gear. Overall, the motor was very impressive and I’d give it an eight out of 10. -SB
PARTS IS PARTS
Split Cases & Clean Up
High Compression Piston Kit
Valve Spring Kit
Heavy Duty Clutch Springs
Engine Plug Kit
High Performance Cam Set
Ti-4 GP Low Exhaust
Team Graphics/Seat Cover
Call 951/738-8050 or visit www.procircuit.com