Set the timing to 7ish with no Vac. I tend to always run more than that but let your engine tell you what it wants once you are driving it.
The factory carb, the Dist vac port is at a position such it does not receive the vac signal till the throttle is opening.

The way this works there is little advance at idle, essentially what the mechanical amount you have set it at.
At a light throttle cruise there is allot of vac to the dizzy offering allot of advance, this is to provide torque and a complete burn cycle for economy.

As you open the throttle more the vacuum diminishes under load, this reduces the advance under high power when the cylinder pressures are high with a dense fuel charge which ignites easier. This reduction in timing reduces the chance for detonation/knock.

With the carb you have, either the throttle plate is open too much at idle or the port is cot controlled by the plate.

Were I would start, connect the vac line, start engine and readjust the idle speed presuming it has increased with the vac advance.

In the past I/we used to need to chamfer or drill the throttle plate to get it where it needs to be for proper control of idle ports etc.

I have no experience with the new China carbs for what they need to run right. I expect these horror stories is simply because carburetor tuning is a lost art.