if i was to buy a uni air filter nd a k & n powerlid for my 400ex would i have to rejet?

 

most likely yes, the power lid is just like removing the lid.

 

First off, forget the powerlid and get an EHS RACING lid. It has an Outerwears over the top. Very slick, and cleans in seconds. I run the EHS lid and a UNI filter on all my bikes.

http://www.ehsracing.com/

Yes, you will need to rejet. Honda jets slightly lean at the factory, to meet environmental standard. Adding more air, will just make it even worse. Running lean can damage an engine, so don't try to skip this one!