i have a fully stock 99' 400ex and my friend is giving me a pro circuit slip on (not sure what pro circuit) but will i need to rejet or should i be fine... head pipe is still stock

 

hi, you should get a jet kiet as well cause now your bike has more of an air flow out you should let more in, by getting a k&n or uni whichever you desire and adding a jet kit will make your quad come alive, its not that exps to do but well worth it..

 

any idea to what i should jet it too i know how to do it but never have

 

if you can try and go for a 144 main, but if theres a quad store close to you try and grab a stage 1 jet kit it comes with everything you need for the install and they come with detailed instructions on how to..

 

i want to go lower? stock 400ex main jet is 148

 

No, dont go lower than stock. If you use real keihin jets start with a 155. Dyojet sells the stage 1 kit. There numbers seem to be a little off compared to the keihin jets. The 400ex has a keihin carb. A good website to check out is jetsrus.