<HTML>Real PITA! (If you know what I mean). Crappy gas no days causes rust. Once it starts, it's a real problem until you fix it. You can install an small inline filter(available
at most auto parts/motorcycle/VW shops - for arount $2.00. It's a clear plastic filter
with a yellow paper filter inside. You should have enough room to install between the
fuel petcock and the carb. What you really need to do is to fix or replace your tank.
To fix it you can remove it and using a vibrator (one made for tanks & the like, I had to
build my own - can send plans later it interested) place polishing media(prepared walnut hulls, marbles, BB's, ball bearings, etc..) inside of the tank and vibrate it turning it on all sides over a 2-3 day period. This will loosen all the rust and polish the inside. You may also have the inside sand or glass bead blasted(more expensive but much quicker). Immediately after cleaning, you need to rinse the inside of the tank with acetone or mek or alcohol and then pour enough Ospo in it to coat all the inside surfaces(top,bottom,sides) when you swirl it around. Pour out excess Ospo(rust inhibitor) and allow the tank to dry overnite. The inside will then be a greyish black color(Phospate primer). After this it will be prep'd to coat the inside with a tank product such as "Kreem". My suggestion is to find a salvage tank since there are plenty of these bikes still around, and save yourself the expense and hassle.(Kreem tank kit runs around $25.00, plus media, plus OSPO, vibrator, etc... I only give you all of these options as I have 38 different bikes and am always picking up another in various need of repairs and have had to make do with what I have(or invent it).
Let me know what you decide!