The big-bore 290F is trying...
The big-bore 290F is trying to save the weight and serve the power. It works.
Big-Bore 250F
We selected a big-bore to represent itself and the ultramodified 250F for a variety of reasons. For one, the performance increase a big bore brings is a great dollar value, and the power is added in the very parts of the rpm range where a stock 250F is weak. Also, nearly any medium-skilled mechanical type can change the piston on a 250cc four-stroke, and it's no harder to install a big-bore kit. All of the 250F models have a variety of bore options with the Honda being the most limited as far as choices go, since the cases won't accept a larger sleeve without modifications. And as long as the rpm ceiling and compression are unchanged, a big-bore should be reliable and run happily on pump gas. A big-bore might even extend valve life, since you aren't forced to scream the engine as much. This KX-F, like the CRF250R, was tuned a bit already. It carried the Pro Circuit pipe from a previous test, and we added PC's revalved suspension to the tiring stock setup. An R&D Powerbowl float bowl with its externally adjustable leak jet cures the off-idle fussiness in the carburetion, a necessity with a big bore. Unlike other hop-ups, this larger piston doesn't diminish the fun character and ease of riding that makes a 250F a blast. The bike doesn't acquire a heavy feel, and you actually clutch it less.
This Athena 290 kit has a plated aluminum bore like a stock bike, and it certainly feels meaty and strong on the track. Surprisingly it doesn't feel as strong as the radar gun says it is. It just feels easier to ride, with more torque way earlier in the rpm range. Mistakes are easier to recover from, and you shift a bit less. You still end up keeping the mill singing to do serious business, but life isn't quite so frantic. Nevertheless, the radar gun shows a clear advantage in the lower part of third gear during a roll-on-not just against the 250F, but running with or slightly stronger than the other bikes, credit that to it just being at the limit of traction, not beyond it as the 250 two-stroke and 450 were. During the full acceleration run the 290 is clearly ahead of the 250F and closer to the other bikes. Yet if the track surface is deep, sandy or muddy, a mod 250F is still between 5 and 15 horsepower behind a two-stroke or a 450.
In terms of weight feel and how much energy the bike takes to throw around, the 250s-two- or four-stroke-have a weight advantage of 9 to 11 pounds over the YZ450F, and some other 450s weigh at least another 4 pounds. While the difference may be relatively small, a 450, due in part to the power and adding the extra inertial forces, feels a lot heavier.
Part of the purpose of this test was to see which bike got around a track fastest, and the two little four-strokes are rarely off more than a second or two a lap when times are averaged. It didn't matter whether the rider was light, heavy, young or older. Expert Chris Dvoracek is a unique advocate of this bike. In the grand prix series he races, 250cc machines compete together regardless of engine type, so the extra boost is attractive, though a 290 is downright illegal. "Honestly, this is my dream bike: a 250F that has that extra little thump. It's a bike I've always wanted but have never gotten. With the extra power I could make a little mistake on the start and still get a top-five start, whereas that would be impossible on a stock 250F." But getting this level of additional power while still keeping the displacement in check is expensive and maintenance intensive.
So what it comes down to is can you capitalize on the corner speed and lightness of the bike? Another huge factor is whether you can pass. Even with the 290, you are merely going to be close on the starts most of the time, only luck or perfection could catapult you to the front of 450s. Passing is easier on a bike like the KX290F, but you'll still have to work for it. The thing is, working hard on this 290 is a bunch of fun.