OK, it looks like your carb could be set up either way, if there's a port about venturi level or slightly below it. If you pull that plug and probe the passageway with a piece of thin wire, you'll probably find that it connects to the bore just above the throttle plates. That keeps vacuum off the port during idle, and gradually applies vacuum to it during part-throttle operation. I looked up the distributor specifications for a 1960 283, and the vacuum advance should start moving with about 4-6 inches of vacuum applied to it. 14-18 inches will give it full movement, so full vacuum at idle will result in about 15 degrees of advance at the crankshaft immediately after the engine starts, even at idle. The ported vacuum won't begin advancing until you tip the throttle in, and by midrange RPM at light throttle, you should have full movement on the advance. Stepping on the gas enough to lower the vacuum below the venturi allows the spark to retard during acceleration to prevent detonation. To get an accurate initial timing setting, disconnect and plug the vacuum line at the distributor while checking the timing.
Jerry
Last edited by Hotrod Lincoln; 10/13/2010 1:53 PM.
