When the idle contacts are closed, i.e., throttle plate closed, the injector pulse time is set by a potentiometer adjustment on the ECU, and is independent of manifold pressure. As soon as the idle contacts open, manifold pressure becomes the main signal to set pulse time. If the manifold pressure sensor has an open or shorted winding in either the primary or secondary circuit, it can cause a shutoff of injector pulses.

To check the pressure sensor, pull off the four-pin flat plug, and use an ohmmeter to test the windings. The center two pins are the primary, nominal value =88ohm. The outer pins are the secondary, nominal =350ohm. Neither winding should be shorted to ground, or to each other.

Additionally, check that the ignition points are in good condition and properly gapped, and that movement of the breaker plate does not interrupt the primary circuit.
While inspecting the distributor, clean the injection trigger contact sets (4) that are in the base of the distributor, under the ignition breaker plate.