The only thing that can cause this in a D-jet motor is unauthorized air getting into the intake manifold. It's not in the fuel side of the equation. You might get small variations from drifting ignition timing, but nothing as much as you're observing. This could be a vacuum leak of some sort, or the throttle butterfly not closing consistently (dirt, fraying cable, plate loose on the throttle shaft).

For a start, I'd get some starter fluid and spray it around looking for leaks.