i was wondering how fas does an stock 05 400ex go, and how fast will it go once i put full fmf exhaust and jet kit on?

 

Like 180mph.

 

lol. For such a popular bike there are many threads already discussed for what you are looking for. People on this site claim a 400ex does anywhere from 65 to 72 depending on weight of rider and terrain. Do some searches for your bike and that will likely get you some answers.