[1QuickS]

You are right, smaller idle air jets will enrichen the fuel mixture as the RPMs increase. The idle fuel jets don't get much air from the idle air jets at low RPMs. If large idle jets were installed to enrich the upper range of the progression then they might be too large for lower RPM operation once the smaller air correction jets are installed.

Also, in tuning these carbs it is useful to assure the throttle plates are open enough to prevent sticking shut (all the way closed, mechanically) but closed enough to block the first progression hole. Most times the idle speed is set with the idle speed stop screws and minimizing the air bleed screws. In carbs with some miles on them this works out well as there is edge wear of the throttle plates which almost requires closing them all the way to keep idle speed down. In carbs that are not so worn or for applications with engines of larger displacement the throttle plates are opened up to provide the air needed but the first hole of the progression circuit is exposed which activates it. In these later two scenarios you have idle mixture supplied from both the mixture screw and from the first progression hole. Setting idle mixture would then have you closing the idle mixture screw and perhaps lead to a leaner idle jet selection but this contradicts the need for a larger idle jet to overcome a lean progression...it gets complicated.

The "fix" to overcome a large displacement engine's need for more idle air (assuming the air correction screws can't provide enough and still allow for idle air balancing) is to drill the throttle plates with a 1mm hole, opposed to the side where the progression holes are and about 3mm from the edge of the plate. (This is easy to enlarge so don't over size the hole and may be closed with solder so this isn't as risky as it sounds.) This hole allows more air at idle to allow the throttle plates to close enough to seal the first progression hole and thereby makes the mixture and air correction screws work as designed.

To determine if the above "fix" is warranted you should remove your carbs, manually adjust the throttle plates to blank off the first progression hole, open the air corrector screws one turn and reinstall the carbs. If you can start your engine and set idle speed and air balance without using the idle speed stop screws to increase idle speed then everything is good, if not then you might consider the 1mm hole fix.

OK, a bit off topic...For a 3.0 with 34mm venturis; typically Webers are jetted with 145 mains and 180 main air corrector jets to match the 34mm venturis and a 55-60 idle jet for a 3.0 engine. Routinely 110 idle air correctors are in the 40mm Webers used in these applications so this is what I'd start with. However the progression hole sizes and arrangements of the Zeniths don't match that of the IDA Webers so I'm not sure what would be a best fit. Your mains seem a bit large and the 185 air correctors may be trying to take away a bit of the upper RPM richness they provide although there isn't much difference between a 180 and a 185.

Once you open this door it becomes a try it and see effort. Changing idle air corrector jets requires removal of the top cover of the carb which isn't as easy as a main jet change.

I have a simple kit that provides a tap, tap drill, instructions and a selection of idle air corrector jets if you like.

I also have a method to drill the top cover to achieve an anti-percolation fix for the Zeniths. Just ask.