We are putting a pro circuit pipe on a 2002 400ex and would like to know what jet size would work with stock air filter and air box.And Later what would be best with a K&N filter

 

I have a Pro Cicuit T4 with a K&N and no air box lid. I am at a 158 Keihn main jet. Your elevation of 158 ft. is close enough to mine to where that may work as a starting point. With a stock foam filter and an airbox lid, I would guess a 152 as a good starting point fo you.

Good Luck.

 

ftwflh

With the same set up as you, I am running a 155 main jet and no other changes and it runs great. I may modify the airbox this year and change the pilot jet, but with the stock filter and airbox, it wasn't necessary.