Not sure what bike you have, but on my 75 750F that I used to have, there was a chronic problem in which the bowls were slow to fill. Tried everything -- the gas cap vent, fuel lines, petcock, in-tank filter -- and nothing seemed to fix it.
Then while fooling around with the carbs, I found there were tiny little mesh and plastic filters that sat right above each of the float valve seats in the carb bodies. A couple of them were really clogged with crap, and cleaning them solved the problem.
I don't hear about those little filters mentioned very much on thi forum, but I can attest that they can be a big mysterious problem if they are clogged. See the picture in this ebay listing -- it's the little white piece at the top.
http://www.ebay.com/itm/like/281061385962?lpid=82