I bought a new WR450 so my brother found a used 01 (I think) YZF426 for a real good price and bought it and that 426 absolutely smokes my 450. Don't know anything about the 426 other than it's pretty loud and looks like it has some sort of aftermarket pipe on it. But it's on a whole different level, in fact it's sick fast. Mine has the free mods done by the dealer and it runs ok but nothing like this 426. I finally got mine out on a big wide dirt road and it has a little stumble right after mid range and then the top end is really flat, just quits pulling. It just turned 120 miles. Do they need some more break in or does it sound like the jetting is a little lean? I ride at about 5000'. Actually I'm glad mine doesn't run as hard as that 426 because it's just plain scary.