Make certain the carb's overflow tubes are clear, and not plugged up by insect nests. Plugged tubes render them ineffective.
Also, check the stand pipes in tha carb bowls to prove they are clear.
EMF stands for ElectroMotive Force. The collapsing magnetic field of the coil generates a voltage in the secondary of the coil for spark. But, as the primary is also within that magnetic field, a voltage is also generated there from the collapsing field, and that voltage travels the wires back toward the points/condenser.
Cheers,