2004 Yamaha 450YFZ Carb Jetting inf?
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Try a Dynojet stage 1 kit. If you really want to tune for best efficiency/power, you might try installing a "wideband O2 " kit. Really helped me tune my AC 500 ,I have an AEM, here's a pic. The gauge on the left. Dynojet's suggestion will be pretty close. I run a gutted exhaust and a K&N and 5 degree timing advance, kept stock pilot jet, used Dynojet's needle and spring, drilled the slide 1/8" and ended up with a Keihin 155 main jet, though I think DJ supplied a jet that worked. Realize DJ, Mikuni and Keihin jets have different numbers.