Dont put it on upside down, I saw a bike here in my town that stopped dead in the middle of an intersection so I stopped to help him, the second I saw the upside down airfilter, I knew what was wrong.
There is always some backwash of fuel that spits out back into the carby and airfilter due to the way these motors operate.
I poured out a sizeable amount of fuel out of the upside housing because it had no place to drip out from because it was upside down
I took out a knife and cut off the bottom tubes, (all four), and made them as flush as possible with the rest of the housing...The filter now was able to fit in the bike frame after those tubes were cut off.
Put it back on for him and told him to have a nice day...It started right up and he was happily on his way.