Quote Originally Posted by Fingers View Post
Disconnect the vacuum hose form the distributor. BOOM! You're in "timing mode".
Also take care to cap/plug that disconnected vacuum line at the carb side, so you won't have a vacuum leak complicating matters while you tune.

The term "timing mode" really applies more to the later injected trucks, which require shorting a specific connector to put the ECU into that mode for accurate adjustment of static timing.