Dual tank question

[raidmagic]

Can someone tell me or link me to an article that explains how the dual tank system works? I have an 80 and only the right tank works. The guy I bought it from told me the left doesn't work and I haven't tried it because I want to look in it first to make sure there isn't a bunch of crap in it.
Anyway what I'm interested in is how the switch over works and what to look for that would cause the left tank not to work. It has a mechanical fuel pump on the engine so I know that's not the issue. What else to look for?

Thanks