You have a vacume leak lower intake manifold by 1&3 ports-pcv feeding oil rich blowby to carb--extra gas is flowing thru gas side of nitrous system causing poor milage and running--probably leak in exhaust system letting in fresh air to gas rich/unburned fuel/oil mix and is igniting in exhaust causing popping.

over comments that lead me to suspect possibility of more vacume leaks------manifold is for quadjet and u have square bore carb plus nitrous plate??????