Hmmm..... that has wondering about something. I just got my bike recently and took the tank off to change the filter. When I removed the tank the only line going to the petcock was the fuel hose. I didn't really look at the petcock that close, but where does the vacuum line go? And where does it get vacuum from?

 

The vacuum hose comes from the left side of the carbs and leads to a locations slightly to the left of where the fuel line goes on the tank. The bike will not run with out this. (also a neat way of draining your gas if you hook up a vacuum too this). I proved a link on this forum to a location that will let you download the service manual. It illustrates it nicely.