Perhaps this is a tecnique used by many but in case you don't you can make a very effective and very cheap air filter for your carb but using a pair of your wifes/girlfriends old pantyhose. Depending on your carb size, cut a square shape from the pantyhose, stretch it over the carb and hold it in place with a rubber O ring available from automotive parts stores.
You can make hundreds of filters from one pair of nylons, just be sure to get permission to cut them up!!

Cheers
Mike