My CBR has almost 50K on it, and as a reward I am going to replace the float needle and seat on each carb. When I clean my carbs, I always check each float needle for wear, but I never unscrew the float seat.
When I bought new needle and seats, I was surprised to see that above each float seat is a small, fine-screen filter. My CBR never came with a fuel filter, and neither the owner's manual, nor the shop manual ever mentioned this screen. If a bike was to sit for a while, this fine screen could easily varnish up and clog.
Here's a picture of the mesh screen, float seat, and float needle, left to right. The screen can just be pulled off and an inline-filter can be installed in the fuel line. When I replace the float needles, I will post a picture of how clean my 16-year-old screens look like.