I have a 400ex with a white bros. ref box and an E-Series slip on pipe with 10 disks.With the pipe on I noticed that their was alot of back pressure, back firing from the bike every time I was going down a hill with out the gas on. So reading the instructions from the pipe I jetted the carb with a 160 main jet. I am still getting the back pressure. I need some other suggestions to help get rid of the back pressure back firing.