When you say backfiring do you mean popping back through the air cleaner or popping crackling out the exhaust?
The pipe at the back of the air box that you blocked is the air injection, witchI believe feeds fresh air into the exhaust ports thru a pair of reed valves in the head and is controlled by the large valve just behind the airbox.
The crankcase breather goes to the front right corner of the airbox.
If too much air is being bled into the exhaust it will mess with the O2 sensors cursing a over rich situation.
Also check for leaks in the exhaust system.