There are sources of rust other than the fuel tank. Most efforts to stop debris from entering the fuel bowls revolve around good fuel filters and fresh fuel lines with a tank cleaning. All these are good but there are other sources to check:
- tank filter screen cleaned
- vent pipes in top cover of Webers rust internally and drop flakes of rust directly into the fuel well; wire brush and apply "Extend" or equal; or remove, replate and reinstall
- spark arrester in OEM air cleaner housing (25mm diameter, right angled tube) is a good source of rust
- also the trough where the seals are installed will collect water, rust and pinhole after time which allows flakes to be drawn into the fuel well (I suffered this one for awhile before I found and fixed it)
- poor sealing of air cleaner housings (OEM and after market also) with sealing gaskets allows other debris to enter your fuel well; apply a thin strip of oil resistant, closed cell, self-adhesive foam
- spark arrester hose may harbor debris which may be drawn into the system
Assuming a well sealed and cleaned air cleaner and the other issues listed are corrected, there will still be crud entering your fuel bowl; the smallest orifice in the Weber carb is the idle jet. Anything smaller than that will either stay in the bottom of your bowl or be delivered to your engine and spit out the tail pipe. Also, the air cleaner operates with a low pressure so it is natural for atmospheric air to be sucked past poor seals and to bring crud with it.