1) If installing or replacing the distributor, follow the vehicle/engine factory service manual instructions (Ford TFI: LINK/GM HEI: LINK). Temporarily disable the "Idle Spark" control (in Idle Settings), so the idle timing doesn't fluctuate rapidly when viewed with a timing light. Without disconnecting anything, start the engine, and turn the distributor to synchronize the engine's initial (idle) timing using a timing light. Ensure there aren't any active 'timing modifiers' at this point (e.g. Timing vs Coolant Temperature, Timing vs Manifold Air Temperature, Knock Retard, etc.). Also, ensure you don't have any erroneously programmed "Warning Enabled Timing Offset" parameters in the "Sensor Scaling/Warnings" (System ICF) or in the custom Inputs/Outputs (Inputs/Outputs ICF). If you're not sure, use the EFI software "Enable Static Timing Set" (in the "Sync With ECU" icon, drop down menu - top Toolbar). When the initial (idle) timing is synchronized, lock down the distributor's adjustment bolt and never touch it again for this purpose.
TIP: If you're unsure about the initial distributor installation/positioning, or you'd like to verify the base timing before starting the engine, it's suggested to temporarily disconnect the fuel injector harness, and crank the engine while viewing the Cranking Timing with a timing light. This prevents the engine from starting, but still allows you to adjust & confirm the base timing (engine safety). When the Cranking Timing is correct (synchronized), the timing at idle will be too.
If you're cranking the engine (with the injector harness disconnected), sync at 15° or whatever your Cranking Timing is.
If you're idling the engine (without disconnecting anything), sync at 25° or whatever your idle timing on your Timing Table is.
