Do you know fuel is leaking by the plunger, or do you just assume it because you see raw gas looking below the air-meter plate? If the source could still be in doubt, You might look into a ruptured pressure regulator diaphram. The safety outlet for the gas then is routed to the breather tube, where it would be sucked into the intake just above the meter plate, in the air cleaner. This would explain its sudden occurance.

Steve