mm:

The above comments would appear to be going in the right direction.
When the throttle is lifted from the idle position the idle signal to the EZL is removed, and less spark retard is applied. If the retard curve of the 420 EZL is not as aggressive as that of the 560 there may be an earlier spark (more advance).
If the distributor is positioned so that the rotor is too close to the previous terminal in sequence in the cap, a spark can occur in the wrong plug.

Try relocating the distributor about 10 degrees anti-clockwise; that will move the rotor closer to the "next" plug terminal, and away from the "last" terminal.
As engine speed increases the rotor centrifugal advance mechanism achieves the same result.