tried something with my raptor last night

I have not had muct time to ride my raptor sicne it been new because of my truck driving job but I have been home for a couple of day. Well i had it out last night messing around with it and decided to try it with the air box cover off and it ran alot smother had better throttle response and mor power. I have noticed a few times b4 i took the lid off it would back fire on decel could mine have came from the factory rich or what