Your 1991 carb set (I own a 1991 -1000F) has a vacuum port on the number one carb on the left side. Your number one carb is the first carb on the left (while seated on the bike looking forward). If your carb set does not have that steel vacuum nipple (changed carb set up over the years happens) on the number one carb on the left side of that carb, then you can cob off one of those top ports that the balancing gauges hook to. Just remove the clip and rubber cap and put a vacuum line form the top port to your petcock vacuum port.
When you go to have the carbs balanced, you will need to add a Tee in with that top port so that the petcock vacuum port will still get vacuum and the balancer can have access to the top four balance ports. So, don't throw that little rubber cap away!!!