Disable or remove you pair valve. If that doesnt work you are going to need a powercommander or similar device. Check the torque on the head bolts. 17in/lbs is the spec. Did you use new gaskets?
Do you have a powercommander?....If so I have a map that WILL FIX the popping/backfiring as long as all of your other things are in order....meaning NO exhaust leaks.
I am running cobra speedster slashdown pipes on mine. If you get a powercommander let me know and I will email you the MAP I am running. None of the standard maps are 100% perfect. It took me a few hundered miles, and the help of another C90 rider, but I got my bike running PERFECTLY with NO backfiring.