I'm with the two filter group. One from the fuel stroage medium and one in the tank. Now if you pump fuel from a 55 gallon drum or other large vessel then one more filter would be needed for the bulk fuel. Filters are changes at 100 hours for the plane, and when appearing soiled at the storage container. I've seen black residue on filters when changing them but they are used in a special application and construction tank and it's likely residue from the tank composition. On my personal planes there's some discoloration when hitting between 50 and 100 hours but it's not a big deal since it's filter changing time anyway.

I have to side with Pe on the vent line being a contaminate culprit. It just doesn't happen and even if it did a good clunk filter negates any issues. Really look at an open vent line sometime. You won't see any dirt higher than a inch or so from the open end. If anyone really believes that any dirt can make it up the tube without touching and sticking to the damp sides of the tube, well, I'll say no more there.

People and paranoia seem to be a match made in heaven. When it gets bad enough someone will come up with a fuel line that was designed to be a full length filter that's cut to an appropriate length to fill the distance from the tank to the carb. What troubles me is that it would probably sell and then the questions would fire up over which is better, a clunk or can filter or a full length filter

Next question. Why does everyone worry themselves to death over getting something in a carb that can easily be removed if they clean the screen filter on a regular basis but they have no concerns at all about what enters the carb throat and goes directly into the engine? Worst case is you can land a dead stick plane but you have to replace a bunch of expensive engine parts destroyed by ingesting foreign materials.

If it makes anyone feel any better put as many filters in the fuel system as you can stuff between the gas can, the tank, and the carb. Don't forget to wash your hands and apply a face mask before fueling to avoid any other outside contaminants.