i have a 2007 400ex. i just put a full ct racing exhaust, k&n air filter, and i run no airbox lid. i went f/ a 148 to 172 main jet. since i've rejetted it has been harder to start and the carburetor pops. if someone knows what the problem is please help me out. thanks.

 

carb pops or exhaust pops?

 

That means the idle circuit is too lean.
Did you bump up the pilot jet to a #42? Wheres the mixtures screw set at?