Well, I'm more confused than ever. From the looks of the vent filters right above this post, it's reasonable to assume that fine dirt is getting into the carb/engine via the Air filter AND the vent hoses.....No?
I've been researching this for a long time and I'm convinced that dirt can find it's way up those vent lines and into the float bowl.
I ordered a tube filter kit from KevinsCycleRacing (dot com) for $20. Cheaper than others I've seen, and he includes one-way valves for 3 lines that will keep out any water that might want to travel upwards during a crossing. He also recommends putting two of the tubes up under the tank to continue venting during a river crossing. Lots of people run them into the airbox, but he says don't do that because the airbox vacuum may affect carb operation. I will post my results when available. I've been getting sand in the bottom of my float bowl all too often. (Yes I have an in-line fuel filter, and yes I grease the heck out of my near-new UNI
filters that are generously oiled with air-filter oil.)
I also think James is spot-on when he says the air filter cannot block all dust. All you can do is keep the filters as clean and as nicely oiled as possible..