Another 308 Carb Issue

Something sounds very wrong here. The idle mixture screws will have an impact on idle speed, but it shouldn't cause a dramatic increase in idle speed. I would think the car wouldn't even run with the mixture screws out only 1/4 turn. Not to be a smart ass, but are you adjusting the correct screws? The mixture screws are #50 in the diagram.

Edit: I see you called them idle air screws. Are you referring to #48 in the diagram? The ones with the locknuts on them? Only one of those on each carb should be open, just enough so the airflow through the two throats are balanced. Two of my carbs have both of those screws closed, with the other two carbs needing one screw barely open to achieve balanced flow.

Other thoughts: There are different idle mixture screw designs for the different series of DCNF carbs. I'm not sure if they are interchangeable. Perhaps yours were changed at some point?

FWIW, I'm currently running #55 idle jets and the car seems pretty happy with the mixture screws out just under 2 turns. When I was running #50 idles, it wanted close to 3 turns.

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Are the air by-pass screws closed? If those are the screws you said are open then I would close them and see what the air flow is after syncing and go from there. I would not set them to a specific number of turns, I would adjust them to get the balance I need.

 

To amplify what Newman is saying: The air by pass screws should not be open at all, unless you need to open one (and only one per carb) to balance the flow across the two carb throats. Doing otherwise is just crutching other linkage adjustment issues, as there are suitable provisions for balancing the two carbs of each cylinder bank, and then the two banks to each other.

When people talk about "3 to 3 1/2" turns out, they are referring to the idle mixture screws. But any specific number of turns out would be just a beginning guesstimate, as the final setting would be dependent on the specific carb version, mixture screw type (long taper vs. cylinder with short taper), available fuel (ethanol or no), and overall engine setup (air cleaners, exhaust, etc).

 

Maybe post a photo of the mixture screws. Mine look like this: Image Unavailable, Please Login

I can't see a one size jet change getting you into a more normal range of screw adjustment. From my notes, I tried #60 jets, and found they needed about 1 2/6 turns out for a good idle. Something else must be off with your set up. Is your timing correct?

Have you followed this or a similar procedure:
https://www.birdman308.com/service/sync/carb_synch_tutorial.htm