[Bored&Stroked]

A/F mixtures at idle are completely controlled by the two idle adjustment needles. Truth be told, I don't worry about A/F ratios at idle . . . I just adjust the idle mixtures and idle speed screws to get the engine to idle as I want it.

Also, the idle mixture screws have nothing to do with the main jets, power valves and off-idle A/F ratios.

Having an O2(A/F) meter is very valuable for main jet selection and even power-valve selection.

Power Valves: If you're running dual carbs, usually you will want to reduce the power-valve selection to a lower vacuum number.

Example: Maybe a stock single carb setup came with a #7 power valve and now (because of dual carbs) you may need to go to 5.5 power valves. The reason is that both carbs will see less vacuum and you don't want the stock power valves coming in too early (at a vacuum level of 7) and causing an over rich situation.

Having a properly installed O2 (A/F) meter will show you your A/F readings at different throttle/load conditions.

Distributor: As others mentioned, you should consider replacing your distributor with one that doesn't care about dual carbs. The stock "load-o-matic" distributor controls advance based on the specific vacuum signal of a single carb and stock engine - with a specific vacuum port off of that single carb (not the manifold). Your advance curve is probably way off, and you can't adjust it with the stock distributor.

I'd find either a good Mallory dual-point or better yet, get one of the converted SBC distributors from Charley NY. His converted SBC distributors will typically have a vacuum advance canister as well - which can help with fuel mileage (if you care about that). Having a distributor with the correct advance curve can make a heck of a difference in overall performance, engine temps, etc.