[Jdub]

Our '78s don't use an air sensor. The popoff valve is not pressed shut by the air filter - it is oriented to allow the cap to open just enough to release pressure and prevent the airbox unseating after an explosion.

The first thing is to confirm you do not have a leak through the airbox. A bad seal at the base of the pop off must be investigated. But the real issue is have you blown the airbox? Try to get a fingernail under any horizontal surface screw - should be no room, no play. Perform a search on airbox and Zimmerman for a start.

Once you know the airbox is fully sealed, turn your attention to why the valve went off. If you do not know that the rotor, cap, and wires are in good shape, get these replaced. A fresh air and fuel filter as well, again if you do not know firsthand they are near-new. You are looking to baseline the car with parts that you'd replace in time anyways. With these parts new, turn to the CO2 setting, valves (tight = bad idle, etc.) with fresh 'plugs. A fully-charged battery is an easy fix and prevents drain on the system (esp. alt. diodes), so that as well. Again, you'd do this anyways and you learn so much. This process removes uncertainty.

Only now should you move to playing with more expensive analysis and parts-swapping. Good luck - it is not rocket science and you will learn a bunch.

EDIT: The dent in the air filter gives you an idea of how fast/hard that top comes up when you backfire.