what are some of the best things to do to an ex 400 so i can go fast

You could always ride it off a cliff... the taller the better, you get more speed that way

Im with Shane-O....

Torch it and buy a two stroke!

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Andrew Warm
WAPAWEKKA
member of SAK

Ok, now that i'm done being a smarta$$, i will give you an honest answer. The things used to make a 300ex faster hold true for just about any engine, be it 4-stroke, 2-stroke, car, truck, buggy, motorcycle, quad, etc. An engine is in its simplest form just an air pump, air comes in, air goes out. To make power, fuel is added to the air, compressed, and ignited. Any thing you can do to make it easier for the air to come in/go out will increase power. So let us start with the simple/cheaper things first
1. drill holes in or completly remove your airbox lid, allows more air in
2. aftermarket air filter(k&n for example) also allows more air in
3. aftermarket exhaust pipe/silencer, allows more air out
Once you've done the above 3, you will need to let more fuel in to go with the extra air. Which brings us to
4. larger jets in the carb.
Thats a good place to start for now, do the above mentioned things and ride and then when you want to go faster again we will talk about the next steps

I added a DMC Dual Exaust to mine and Also a White Bros CDI ignition module supposed to raise the rev limiter to another 1000RPM.