What is the function of the vacuum hose connected to a vacuum-operated fuel petcock (fuel valve) on many underbone motorcycles?

1. It uses engine intake vacuum to pull a diaphragm open, allowing fuel to flow from the tank to the carburetor only when the engine is cranking or running.
2. It sucks excess fuel from the carburetor back into the fuel tank during deceleration.
3. It uses fuel tank pressure to force air into the carburetor float bowl.
4. It lubricates the petcock valve mechanism using oil vapors from the crankcase.