I have a 1996 CBR 900RR and have put new spark plugs in and now it backfires when I let off the throttle and the motor is winding down. It also does it when in neutral and seems to be revving high when I let off of the throttle. Could this be related to the air filter? Because it doesn't look like it is sitting on carbs correctly. Could it be taking in too much air?
You've likely got an air leak between the airbox and carbs. Check all the plastic/rubber fittings are seated correctly, and any pipes/hoses are where they should be. Also check any rubber tubes or ducting etc for cracks, rubber perishes over time and can split easier than when it was new.