Great advice; I'm using a new, clear fuel line, so there isn't the internal filter in there (though I do remember seeing a super old one in the last line I used). I'll make sure the 2nd inline filter gets removed, too.
I did the vacuum test by pulling the vacuum hose from the carb and confirmed that fuel flowed through the line from the tank, so it's not the petcock. @
ducatiman confirmed this earlier in his tests on the petcock. This means it has to be insufficient vacuum from the carbs, right?