what can i do to make my 07 400ex faster without touching the engine? its stock except for a white bros e2 pipe. thanks in advance

 

The pipe is designed to let more air out, but if you aren't also letting more air in...what's the point??? Take your airbox lid off, and get a ProDesign (ProFlow) adapter and foam (made by UNI) air filter (I don't trust K&N!)....and of course a rejet!

 

Drop some weight. That is a good place to start. It is all about power to weight ratio. The lighter the quad the faster it will go. Maybe invest in some lighter rims and tires for less rolling resistance.