Exhaust and jetting basics

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.

It's not at all clear, but I'm going to assume that your first question is whether the first two sentences are true or not. The answer is "sort of".

The header will not have a very great effect on jetting one way or other unless it's very different from the original in place at the time the jetting was set up. Likewise, a dent won't do much unless it's at least 30% of the header diameter deep, and then most people won't notice. Packing? Maybe a little more of an effect if you compare freshly packed to all of it gone. Some of these conditions do affect performance, but not by changing AF ratio, nor can their effect be reversed by rejetting to compensate.

The '06 header uses the same flange that was used on earlier 450's and thus has the prefix 5TA stamped on it. 5JG is a prefix associated with the YZ426, but the number stamped on the flange is only an indication of what motorcycle line the flange was originated for, and not the whole pipe. In short, there's not much way to tell by any markings on a stock YZF header which one it is.

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