backfire and bb's on tin roof

Please tell me that you retard your timing when you use the nitrous?! 12 or 13 degrees btdc is fine, but if it's still pinging you need to back it off until there is no detonation. You should retard the timing down to 8 or 9 degrees btdc when using the nitrous. But check your plugs too, like KiltedBanshee said.