There are three engine signals that cancel the power to the retard solenoid. The throttle position sensor cancels the power when the throttle is opened past idle speed. The AC cancels the retard solenoid power when AC is switched on. The 100C engine temperature sensor cancels the retard solenoid power when engine temperature exceeds 100C. The retard solenoid only receives power to retard the spark at idle speed and only idle speed.

If you want your engine to run better, set the timing at 30 degrees advance at 3,000 rpm without the vacuum line connected. Re-connect the vacuum line and reset your idle speed to the book value. You will then have a smooth even idle and also when the AC kicks on or the temperature exceeds 100C your spark will still advance slightly to raise the idle speed and compensate for the higher AC drive loads or to lower the rising temperature. If you fail to use the retard feature then your engine will slow down and may stall at idle when the AC is turned on or it gets hot.