Important Note: There are usually two vacuum ports on a Quadrajet. One is a ported vacuum port, and the other is a manifold vacuum port. The ported vacuum port is the correct one for the distributor's vacuum advance. This port only provides vacuum when the throttle is open, thus providing advance only when needed during acceleration. Using the manifold vacuum port will result in incorrect timing.
To be absolutely sure:
1. Visually inspect your carburetor: Look for two vacuum ports. One will likely be larger than the other. The smaller one is usually the ported vacuum port.
2. Consult a carburetor diagram: A diagram specific to your Rochester Quadrajet carburetor will clearly show the location of the ported vacuum port. You can often find these online by searching for "[Your Carburetor Model Number] diagram". The model number is usually stamped on the carburetor itself.
3. Consult a shop manual: A factory service manual for your 1970 Pontiac GTO will provide definitive information.
Using the wrong port will negatively impact performance, fuel economy, and drivability. If you're unsure, it's best to consult a mechanic or someone experienced with Quadrajets before connecting the line.
