I also have the '00 YZ426. From my personal experience and following Rich Rohrich's advice, I started using VP C-12 and took the time to jet the bike properly. The rewards have been nothing but extremely satisfying. C-12 typically calls for slightly leaner jetting than pump fuel. If you are willing to do the jetting, it's definitely worth it. Since it has turned to warmer temperatures outdoors, I'm running a #158 main jet and the engine totally rips! I'm using the stock pilot jet at 1 3/4 turns out. My engine never stumbles, hiccups, hesitates, or gets hard to start. Throttle response is phenomenal. :cool: