There has been some very bad misinformation posted on here about having some of fhe transfer slot expose to make the carbs idle good, WRONG
The slots should not be exposed at all below the throttle plates, if they are your carb is not idling on the idle circuit only due to extra fuel being sucked into the motor from the transfer slots
I had to find out the hard way that opening the throttle plates enough to expose the transfer slots also allow the power valve circuit to start dumping extra fuel into the carb on low vacuum motors at idle speed with higher rated power valves in the carbs, motor had less than 6.0 vacuum at idle with a 6.5 power valve would load up and die until I increase the idle speed to make more vacuum than 6.5 inches of vacuum at idle speeds
Years ago someone, maybe Holley themselves, started drilling small holes in the throttle blades to allow extra air into the motor to avoid opening the throttle plates enough expose the transfer slots
Try removing the lead plugs blocking the outboard carbs. idle circuit mixture screws in the base plates and gently close them up and back them out 1/3 to 1/2 turn only and see if that helps on your problem
You can figure this out and fix it