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.
Then it was time to see what she would do. The engine doesn't really feel faster at high rpm, but it's strong and responsive at every other rpm. The best way to enjoy the engine is to purposely not downshift for turns. The 290 pulls through strongly then catches fire on the way out of the corner. That sort of power is awesome for a heavier rider like myself. I wasn't sure a faster rider would feel the same, so I had Jonty Edmunds try it. Jonty is more of a 450-size guy, but he has youth, fitness and the talent that I don't.
After a couple of motos, Edmunds had this to say, "I thought the 290 was great. I prefer 250Fs to 450s because less power is more fun. Having said that, 450s have an advantage because you can be lazy, but if you try to ride them hard, they can kick you in the ass. 250Fs are more fun but you have to try harder. The 290 was the best of both worlds. It allows you to be a little less on the ball as far as being in the right gear all the time because of the additional cc giving it a midsize four-stroke feel. You can also ride it hard and it won't get away from you. Sometimes, what takes away a bit of the fun factor of the 250Fs is when you're riding them hard, in high revs, the handling is compromised a bit and they get twitchy. In the midrange, the bikes just handle better, and with the 290, you can do just that: Keep it in the midrange. It started easily and the carburetion was perfect. In many ways, it's the kind of bike I wish manufacturers would produce. I think it'd make a great woods bike with the kind of power it has. Progressive, smooth, strong but not too strong."
Jonty and I are in age-group classes, so a 290 is legal. I was curious what a younger, lighter rider would say, so I dangled the bike like a carrot in front of Chris Denison. And, again, it was a hit. "Once I got the suspension dialed in, I was super pumped on how the 290 worked. The added power gave the motor the lugability of a bigger bike without feeling overly powerful (or lethargic), and the capacity to ride a gear higher everywhere was outstanding. Considering how little this mod costs, I'm pretty surprised that big-bore kits aren't more popular."
All things considered, I'm delighted with the 290. It isn't perfect, since the engine-braking increased and on full-traction uphills like we find at Piru MX, the engine feels a little reluctant to rev. On the other hand, for every other track I felt no negatives at all other than learning to delay downshifts to lessen the effect of the engine-braking. The jetting isn't odd, starting remains easy, it doesn't run hot or feel finicky. Athena calls for a new piston at 15 hours, but I plan to replace it at 20 hours. That's when I'll finally be able to give the 290 a total thumbs-up. In the meantime, I'll just enjoy the heck out of running up the hourmeter. It's a tough job, but I did volunteer.
Parts List
Athena YZ250F complete big-bore cylinder kit
Acerbis front and rear fender plastic
Factory Effex Evo 5 graphics kit