Do I need to rejet?

[CrewCab]

Purchased a FMF Powercore4 slip on and wanted to know if rejetting is needed. It will be going on a 06 Honda 400ex with stock airbox and headpipe.

[MTDman11]

pretty much everytime you change exhaust you have to jet. i heard from other people at fmfs website the have recomended jetting specs for each pipe

yes you will definetly need to rejet.