always best to run an airbox as it makes life so much easier, but sometimes on a build theirs no room for one or if its a tuned motor. if you are after a good standard running bike stick with the airbox.
I use a tin of carb cleaner with a straw on the button, when you spray into the pilot circuit you can see where the fluid comes out on all 4 carbs, if one isn't flowing the same as the rest then you know where the blockage is. works for me.
they don't seam to be available ???? but hears the P.N.
AIR FILTER for Suzuki GSX-G 1100 1992 # SUZUKI MOTORCYCLES - Genuine Spare Parts Catalog (bike-parts-suz.com)