I have a 400 ex and I just bought a new slip-on pipe (FMF power core 4 square) and re-jetted the bike from a 148 main jet(stock) to a 158 main jet, and I installed a UNI air filter. The bike runs and sounds great and very responsive, but the engine and pipe seems heat-up much faster and hotter than normal. I did not alter the air box any (yet), I did some reading on the air box and people are suggesting to drill 1" holes on the lid to increase the air flow. I am mainly concerned about the engine heating-up, is that normal, if not what should I do about it and what suggestions do you have for the air box lid?

 

That could be normal I'm not sure. For the air box lid I would just take it off.