'77 kz1000 Carb sync. Cylinder #2 unsyncable.

I think I see the problem. I believe the "vent" on the #3 carb is actually a vacuum fitting (see diagram below). Try capping it off and then see if your vacuum readings improve. I see that your original readings show #3 as low vacuum which makes sense if that open fitting actually is a passageway directly into the carburetor venturi. Ed
Original Readings:
Cylinder#1- 23 cmHg
Cylinder#2-21 cmHg
Cylinder#3-10 cmHg
Cylinder#4-21 cmHg

Success! I had a feeling that it was going to be something little like that. Just goes to show, never trust the previous owner. In my case the PO was a motorcycle mechanic and had all 3 ports vented to atmo.

Thanks for the help guys. Appreciate it.

Rage