One thing I have run into when I have a low vacuum reading, is that the throttle cables are adjusted incorrectly, and holding the slides open. Also the throttle stop screw may be turned too far in, holding the slides open. At idle the slides should be almost closed causing a the vacuum reading to be up in the normal range.
I would start with a bench synch. That is eyeballing the slides and making sure they are about the same amount open.
Another thought is that, did you rebuild the carbs? Did you pull the slides out? I have made this mistake and put the slide in backwards. The "curved" side of the slide should face towards the rear of the bike, the intake side. The "flat" side should face the engine.
Once you bench synch them, see if you have readings on the gauges. You will need to fine tune the air/fuel mixture screws so that it idles correctly, at the same time playing with the throttle stop screw. The rpms should drop quickly, if not the throttle stop screw is turned too far in and the slides are being held open.
I hope that made sense, but always make more sense once you've done it a few times. Let me know if I can clarify anything.
