What is the idol mixture settings on a 1980 stock Chevy 305 carter carburetor?

There isn't a single "idol mixture" setting for a 1980 Chevy 305 with a Carter carburetor. The idle mixture is controlled by two screws, one for each barrel of the carburetor (it's a two-barrel carburetor). These screws are usually located on the side of the carburetor, often near the base. They are often marked with an "I" or "Idle" or sometimes are just obvious by their location and small size relative to other carburetor adjustments.

There's no universal setting. The correct idle mixture setting depends on several factors, including:

* Altitude: Higher altitudes require leaner mixtures.

* Engine condition: Worn engine components might necessitate adjustments.

* Air filter condition: A clogged air filter can affect the mixture.

* Vacuum leaks: Leaks in the intake manifold or vacuum lines will throw off the mixture.

How to find the correct setting:

1. Warm up the engine: Let the engine run until it reaches normal operating temperature.

2. Find the idle mixture screws: Locate the two screws on the carburetor.

3. Initial adjustment: Start with the screws turned in roughly 1 1/2 to 2 turns out from lightly seated (this is a starting point only).

4. Adjust one screw at a time: Slowly turn one screw in or out, while observing the engine's idle speed and smoothness. You're looking for the smoothest possible idle speed without hesitation.

5. Repeat for the other screw: Repeat the process for the other idle mixture screw.

6. Fine tune: Once you've made initial adjustments, make small incremental adjustments to both screws to achieve the best possible idle quality. You might need to alternate between the two screws several times.

Important Note: Improper carburetor adjustment can damage your engine. If you're uncomfortable adjusting the carburetor yourself, it's best to take your vehicle to a qualified mechanic. They have the tools and expertise to do it correctly.