Why is my shallow well jet pump cycling on and off rapidly?
Listed reasons for excessive cycling in J+ installation manual:


Defective suction check or foot valve
Water logged pressure tank
Pump farther than 5 feet from tank
High friction loss valves between pump and tank. (Use only fully open gate valves)


Parts breakdown list



I take it this is a shallow well with the jet assembly bolted to the front of the pump and not a deep well installation with two pipes coming out of the pump with the jet assembly down in the well.

Jet pumps work by blasting a high speed jet of water through a venturi to augment their suction. If your crusty fitting was in the opening (4), it could release chunks that get caught in both the jet nozzle (5) and the gap between the jet and the venturi (6). The plug (1) allows you to use a piece of brass welding rod to try chunking out anything caught in the jet nozzle.