Yes the carb-to-petcock vacuum tubing is connected.
It seemed that it leaked from the fuel T between the middle carbs, so I will try moving it around. The problem may have appeared when I jerked around to take off the extra gas filter mounted between the rubber tube that goes down to the T and the rubber tube to the petcock. I had to jerk rather hard to get it off.
If moving the tee around to reseat the o-rings doesn't work, is it possible to change to new o-rings without removing the carb bridge and taking the middle carbs off?
I also thinking that maybe something in one of the middle carbs had got stuck by old gas in the system. I emptied the tank and run the engine out of gas before the winter.
Regarding the petcock I may take it apart and clean it. You're mentioning that I could run on PRI, as long as I don't forget to put it to ON when stopping the engine. One strange thing is that when I was out with the bike and had it on PRI, the engine died as well. I still had gas. So I thought it was just the reserve left. Since the vacuum problem also appear wit the petcock set to RES, I added som more gas an, set it to PRI and, guess what, it started again. Thinking about it, you're right, having it set to PRI, it should be possible to run out of gas since it also includes the reserve. STRANGE! I will have to investigate this.
I intend to add a carb system cleaner to the gas to resolve any bad old gas leftovers. This would perhaps clean the petcock as well. What do you think?
Beside the problems, it felt so niiiiice riding the bike again. Nothing like the first ride in the spring, right?