400ex header vs stock manifold differences
 

is there really that much difference the only real thing i notice is the header pipe diameter is a 1/4" bigger . thoughts?
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just that little bit larger i think will let it breathe a lot easyer, but this is just me talking out my ass

If you have the bigger header put it on and rejet. The more engine and intake mods you have the more gains you will see from the header. If you ex is stockish you will see some gains but nothing huge. IIRC the power bomb header was one of the better flowing headers for the ex.

i got his pipe on ebay for a hundred bucks with header so i grabbed it. what jet kit do you recommend? bike is bone stock minus pipe

If you want a little more get a uni air filter and crack the air box lid. Then start to rejet. Jetting is required and a PIA. Its all trial and error. There are many variables like temp and elevation. I would start with a 160 and try it from there. BTW buy keihin jets, same brand as the carb.

probly going to go with a k&n with pre filter, every one has their preferences. this bike is going to be for an mx track, not racing it just learning to jump. i like my grizzly for trails and mud. thanks for advise