Runs on all four but unstable idling

[Hansmann]

Hello Forum
I haven't been using this forum much, but I have followed the forum as an interested reader of all the users' technical problems. But now, I hope that I can reach out for some assistance. I purchased a Standard GL 1100 from 1982 a few yrs ago. The bike is well kept, with some age-related patina. I enjoy the look and the laid back style of the bike. Now that the new season is knocking on the door, I am eager to take it out on the road again.
However, I have one issue that I cannot solve myself, and I hope some on can help me to the proper solution to fix my GL 1100 1982.
Last year at the end of the season, my bike began to run erratically, and I diagnosed the issue likely to be related to the carburettor since they, after all, are almost 40 yrs. old and looked like they have never been cleaned.
I pulled the carburettors, and because I am not a trained mechanic, I decided to have them shipped to a professional rebuilder. At the same time, the valves were adjusted, although not much off. I also added a new fuel filter, spark plug less than 500 miles, fresh oil, and new throttle, and a return cable. The carburettors came back and installed.
The bike fires up, and it runs on all four but unstable idling. I had it for a spin before I put it away for the winter. The bike drives bad at rpm under 3000 and much better above. This tells me that the balancing is off. But when I attach my carburettor balancing tool, it is impossible to get a stable vacuum on any of the four cylinders. This direct me to a vacuum leak somewhere. The vacuum hose on the no. one carburettor is attached, and I am now out of suggestions on where to continue. Maybe someone could help me in the right direction
Kind Regards,

[dingdong]

Balancing the carbs will only affect the idle. Generally has nothing to do with runability above idle.. You won't get a stable vacuum pressure when using your vacuum gauges. it will fluctuate rapidly and that is normal. You just need to get all 4 to an equal reading.

[Hansmann]

I might have been unprecise. The vacuum is very low on all four and is largely unaffected by the vacuum adjustment screws.

[Chewy]

in order to get an even reading on you vac gauges, you will need to put some type of cutoff valve inline to the gauges. start the bike and slowly close the cutoff valve until the gauge needle barely jumps. then you will be able to see how far off the carb synch is. i got lucky when i had mine rebuilt Pistol Petes son, Mark, rebuilt mine. i was one of the last customers to have a set rebuilt by him.