A weak or even missing spring would, in fact allow runaway gravity fed fuel flow, (yes, a spring "stretch" can correct this) but only a torn, pinholed diaphragm would account for fuel behind it..... into the vac section, allowing unmetered fuel/air sucked in by the engine.
that is (was) @
dodgerdad stated condition, fuel in the vac line.
The 2 subsections of the petcock, the front end (fuel) and rear end (vacuum) are distinctly separate sealed paths....once violated, correction is mandatory.
In any case, many problems are associated with these vacuum type petcocks as the bikes begin to age, accentuated by long periods of disuse, ethanol fuel and even outdoor storage (open to atmosphere thus accelerating internal corrosion).