Along with jetting, your exhaust must be dialed in.Because any jetting changes or changes to your exhaust (packing changes, dents removed, pipe gaskets) will make your previous work worthless. If it is true, here is my question. My stock YZ450F runs differently with a different stock header pipe than a different year stock head pipe? The stock '06 head pipe P/N is 2S2-14611-00-00 the Flange is stamped Y-5TA. The other pipe is of unknown year and the flange is stamped Y-5JG. any kind of info would be a help or just decoding the flange numbers would be a help.