Vacuum Advance hookup - Firewall module?
That is an OSAC valve. "Orifice Spark Advance Control". Nominally, its job is to delay vacuum to the vacuum advance, thus reducing NOx emissions from the engine and allowing the new 1973 models to squeak past their Federal emissions type approval tests so they could legally be sold. In fact, that's not actually how they worked. They actually worked by spoiling the driveability of the car: they created a hesitation/stumble so severe and dangerous, and took such a large bite out of acceleration at anything below full throttle, that you simply choose not to drive the car…thus reducing exhaust emissions!
Leave it bypassed. Hose directly from the one and only correct nipple on the carburetor directly to the vacuum advance, without stopping anywhere on the way.
(Starting in 1974 the OSAC valve was moved to the air cleaner body)
That is an OSAC valve. "Orifice Spark Advance Control". Nominally, its job is to delay vacuum to the vacuum advance, thus reducing NOx emissions from the engine and allowing the new 1973 models to squeak past their Federal emissions type approval tests so they could legally be sold. In fact, that's not actually how they worked. They actually worked by spoiling the driveability of the car: they created a hesitation/stumble so severe and dangerous, and took such a large bite out of acceleration at anything below full throttle, that you simply choose not to drive the car…thus reducing exhaust emissions!
Leave it bypassed. Hose directly from the one and only correct nipple on the carburetor directly to the vacuum advance, without stopping anywhere on the way.
(Starting in 1974 the OSAC valve was moved to the air cleaner body)