Anyone want to collaborate on a GPz550 runs like crap flow chart?

If you don't mind including my LTD 550, then I'll chime in.  I'm getting about 27MPG, which is about half of what I'd expect.  I've gone back and synchronized the carbs once again hoping it does a bit better.  I also get a sound like a chain rattle when cold at low RPMs.  Normal?  I've heard yes and perhaps no.