carb question (Holley)

More timing = throttle blades being closed more = less air flow by the boosters.

Timing will most definitely affect the amount the blades are open to maintain a certain idle rpm. Not enough you can and will get booster draw at idle. Seen and fixed this many times with the total timing crowd. Try timing a car with initial set at 0* and then again at 14-16, those two initial setting have VASTLY different carb idle setting profiles.

Here's what I would do for the OP's deal.
Take the carb off and make sure the primaries are not open further than a square showing for the transfer slots. Google it if you need to see what it is.
Verify that the secondaries are closed up.
Set initial timing at 14-16. D this with the engine OFF, ignition on. That way RPM has no effect.
Figure out the booster dumping fuel. This may cure itself once you reset the throttle blades for the transition slots. Float level, sinking float, etc.

Plug everything off that has vacuum at the source, carb, intake manifold etc... not at the end of a hose. Isolate the carb, intake tract from anything peripheral.