Where - 65 gto 389 carter afb auto engine used with 4 speed is there a vacuum port to use for the vac advance timing set 6 btdc line off attaching jumps 15 where a?

There isn't a single, universally agreed-upon vacuum port on a 1965 GTO 389 Carter AFB-equipped engine with a 4-speed manual transmission to exclusively power the vacuum advance for the distributor. The location depends on the specific carburetor and intake manifold configuration. The distributor itself might even have dual vacuum ports, one for ported vacuum and one for full manifold vacuum.

Here's why it's complicated and how to find it:

* Carter AFB Variations: Carter AFB carburetors came in numerous variations for the '65 GTO. They had different porting and vacuum configurations.

* Intake Manifold: The intake manifold also influences available vacuum sources.

* Distributor: The distributor itself might have a port for ported vacuum (vacuum only at a certain throttle position) or manifold vacuum (constant vacuum). The ideal source depends on which your distributor uses. Many prefer ported vacuum for crisper throttle response, but this needs to be compatible with the distributor.

Where to look:

1. Distributor: First, examine your distributor. Does it have one or two vacuum ports? If it has two, usually one is for ported vacuum and one for manifold vacuum.

2. Carburetor: Look closely at the carburetor base. There are usually several small vacuum ports. Some might be plugged.

3. Intake Manifold: The intake manifold may have vacuum ports as well. These are less likely to be used for distributor advance but could be used.

4. Ported Vacuum vs. Manifold Vacuum: This is critical. Ported vacuum provides vacuum only when the throttle is opened; manifold vacuum provides vacuum all the time. A wrongly connected source can cause poor drivability, especially at idle or low speeds.

How to determine the correct port:

You'll need a vacuum gauge and a vacuum pump or source. Test each port individually.

* Manifold Vacuum Port: A manifold vacuum port will show a steady vacuum even at idle.

* Ported Vacuum Port: A ported vacuum port will show near zero vacuum at idle but increase vacuum as the throttle is opened.

Recommendation: Consult a 1965 GTO shop manual or a Pontiac-specific forum. These resources often have diagrams showing vacuum routing for specific engine and carburetor combinations. Getting this wrong could lead to poor performance and drivability issues. Using a vacuum gauge is essential for proper identification.