what would cause my 400ex backfire out the exhust, it has 440 big boar and white bro header and exhust.

 

most likely the jetting.when does it do it upon deceleration?

 

A lean condition can do this. Was it recently rejetted??? Perhaps the jetting isn't spot on.

If it was running fine, then began doing this spontaniourly, it could be a leak in the intake boot, a leak in the air filter, or a clogged jet (idle jets in particular clogg up easily).