Hey! What happened to my post? Here's a repost of the main info: I use a U.S. Wr400 pipe on YZ426 for organised/public trail rides, dropped needle 1 notch, sized main down to next smaller that came with bike, left pilot and idle settings same, bike starts great runs fine and has never "fouled" (read carboned) a plug (leave muffler insert in, or else don't bother putting the WR pipe on, it'll be as loud as the other pipe, BUT the WR pipe is waaay heavier than the YZ pipe, but you won't notice when trail riding anyway). It will not hurt the engine. It's the opposite of jetting up for a less restrictive exhaust, and you will lose some top end, which you don't fully use when trail riding anyway.
Works great and you'll get big brownie points for being a quiet guy. You'll never get your YZ up to full revs/full throttle in the woods anyway, ok, well one time after which you'll be in the hospital, unless you're really a pro who's teasing us by pretending to be a newbie. ;)