A problem I noticed on this bike when I first looked into it a few years ago was that the little "mounting arms" between the carburetors (the arms that hold the two carbs together) were broken. The carbs, which are supposed to be "fixed" relative to each other, were actually free to move about. Not good... especially with regard to the progressive linkage.
I looked up "broken carb mounts" or something to that effect on the main XL600 thread, and I learned that this was not an uncommon problem. However, I didn't really notice any particularly awesome solutions. My first solution, a hose-clamp around the float bowls, was not awesome either. So I came up with the brackets seen in the following two pictures:
In the bottom picture you can see that I have sandblasted one float bowl. At the time of this writing, the carbs have been totally sandblasted and wire-brushed and look like this:
At this point they are done being rebuilt, with the exception of new float bowl gaskets, which I am waiting for in the mail.
I actually took this last picture for the purpose of asking a question... which I have posted here
in the XL600 thread.