The priming question has been answered

However on the mag drop----

If there is a bad or fouled plug, you will see a drop as soon as the
good mag is switched off.

If however there is a timing error which can be caused by the points
wearing, or the magneto itself slipping position due to loose hold down
nuts---then the drop will be much slower taking several seconds to
stabilize.

If you switch too quickly during the mag check you might miss this.

Cheers:

Paul