The slightly faster gearing, Maxxcross tires and new Renthal bar were all an instant hit, but I hit a virtual wall when it came to the exhaust. Jumping right to the titanium full Leo Vince system, golf partner/testing homie Ryan Orr and I were less than stoked on the changes to the power. A great supercross pipe, the full system gave the bike a quick-revving, narrow-windowed power that didn't work well on any of the three outdoor tracks that we rode. Feeling the poopy-face coming on again, we were happy to find success with the stock header/ Leo Vince stainless steel slip-on combo, although the broadened power and better mid was accompanied by boggy response and sporadic hesitation. The golden ticket ended up being in the spark arrestor tip, which calmed down the throttle response issue while only slightly suppressing the newfound power, ultimately adding more meat on both ends and boosting the midrange considerably over stock. We rotated through all of the combos-including stock-once more to make sure we were happy, and then settled upon the slip-on can with S/A screen as the final choice.
In my opinion, the revised RM-Z250 makes for one heck of a race package. By stepping up the suspension performance, opening up the cockpit, slightly tweaking the gearing and broadening the power throughout, we transformed the entire feel of the bike. What's more, I didn't once have to remove the motor from the frame or crack the carb to fiddle with the jets. All of these mods were completed without breaking the bank, and thanks to the spark arrestor, the bike is now much quieter and-best of all-off-road-able. You know that face that people make on the 4th of July whenever a huge bundle of colorful fi reworks explode in the shape of a massive, awesome peace sign? Well, I'm making that face now.
Parts List
Leo Vince X-3 stainless slip-on exhaust
Throttle Jockey graphics and gripper seat cover
Renthal 997 Twinwall handlebar, dual-compound half-waffle MX grips, 47-tooth rear chainwheel
RG3 linkage tie arm and cam with bearings, front and rear suspension revalve
Twin Air Dual Stage filter
Tires
Maxxis Maxxcross IT front and rear tires
Hey Big Boy
Modified Yamaha YZ290F
This is how it works at Dirt Rider meetings: Jimmy acts as the moderator,sort of keeping the welter of ideas and comments somewhat on track toward the eventual completion of a magazine. And while he easily could, he doesn't simply grab up all the cool/fun ideas and stories for himself. He does pull rank on some, but many others are sort of thrown out there like a dolphin trainer hucking smelt to see how many tricks we'll do to get it.
During the discussion of modified 250Fs, Jimmy mentioned in passing, "We should probably do one as a big bore." I'm trained like Pavlov's dogs-say big-bore 250F and I start to drool. But I didn't let the involuntary salivation slow me. With speed that would dazzle a game-show host, I slapped the table and claimed the big-bore bike. We were supposed to be picking a machine that had specific problems that bothered us, but all of the 250Fs are fun, and I love to ride any of them. But for a person my size, they can all stand more displacement.
Since I was looking for a big-bore candidate, the Kawasaki, Suzuki and Yamaha were the most appealing. Each will take a larger bore without any other modifications, so I was hoping for one of those. Jesse jumped in and claimed the Kawasaki and Denison was already started on the Suzuki, so that left the Yamaha. But I didn't see that as a third choice. I get along well with the Yamaha, and the engine has been around for many years, so there are plenty of bigbore kit options available. We were looking for a reasonably priced option that was somewhat in line with the price total for the other bikes. And personally, though I've never had a problem with an iron sleeve, I wanted a kit with a nikasil bore and aluminum sleeve like a stock bike has.
A little checking revealed that Athena had a well-priced kit that included its own brand cylinder that increased displacement to a whopping 290cc! The kit comes complete with everything you need for around the same price as a complete exhaust system ($756.77).
Installing the kit was easy. It comes with the Athena cast cylinder, piston, rings, gaskets and circlips. The job was as simple as changing a stock piston. Athena includes directions, and since I had never rebuilt a Yamaha on my own, I was happy to have the included manual. The only "special" tool I used was a torque wrench. The cylinder surface looked very shiny and I was tempted to run a hone through it, but Athena reps said no. I rebuilt the top end of the engine, installed graphics, prepped the bike and wrestled on new tires during a single day in the garage. Except for small letters on the cylinder and a notably deeper growl, there's no evidence that the bike is anything other than a stocker. The stock plastic was looking tattered and scratched and it had white marks in the blue. I contacted Acerbis and replaced the front fender with a blue one, then added a white one on the rear. Most of the white marks were on the rear fender, so it seemed easier to switch than fight, and I like the way it looks.
When it came time to break it in, I headed to the I-5MX track with a couple of friends. They followed me to the track with 450s and I thought, "This shouldn't be fun for them. During break-in I won't be able to go fast or do the jumps." Surprise! With 290cc on tap, the YZ handled every jump on the track that I normally do and all without ever opening the throttle past half-open. I gave the engine three good rides that got it good and warm without lugging it hard or spinning it unmercifully, then let it cool in between.