I've never had a backfire yet. When I first started using nitrous I had a BIG problem being too rich and running too little timing. I was trying to do my liscense passes and the car was stepping into a wheelstand like it was walking up a ladder! It would pop and fart out of the exhaust and you could see the car making small steps up, I could feel it in the car too, absolutely crazy! Never came out the intake though. I eventually found the right people to talk to and got it right. No more problems. Now when it leaves it smoothly pulls the wheels up and smoothly sets them back down, really amazing how gentle it is, even with the front end 4 feet off the ground!
Sorry, can't help you with the backfire issue.
I also hope that you aren't one of our Wedneday night guys that I just love to see at E-Town for the test-n-puke sessions! I love the guys that purge about 2lbs in the air, then bang it through the motor, go to the line, and every once in awhile launch the hood scoop 40ft in the air with a HUGE fireball! It's a good show for us in the stands whil eating our burgers though.