jetting

[michigander]

Hello,i got a 400ex put a R4 meg full exhaust on ,and a white brothers high flow air cleaner on .runs fine except back fires durring shifting,spark plug is black.Anybody that can help me out would be great. THANKS

[northeast400]

did you change the main jet yet?

[michigander]

not yet

[cals400ex]

i would go up a few sizes on the mainjet. also, try a 42 pilot jet. the pilot jet should reduce the backfiring. with the 42 pilot, run the fuel screw at 2.75 turns out and see how that works for you.

[northeast400]

A guy I ride with has an R4 this is set-up
155 main
stock airbox with lid on
twin air filter
2.75 turn on the air screw

this should give you a good starting point

[michigander]

thanks guys for the help