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Originally Posted by mole2
The generalization that all "modern" petcocks are vac operated is untrue. I just looked at my 2009 carb V-Star 650 and my neighbors 2007 carb V-Star 1100 and both do not have a vacuum line to the petcock. They both have electric fuel pumps which draws the fuel. Hence the petcocks are flow valves with Off - On - Reserve. Teaching AND utilizing the "turn the petcock off" (on bikes/petcocks that have an off position) is very applicable to bikes made today.
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Typically the pump has a built-in pressure-relief valve to keep it from oversupplying the float bowls. When the float bowls fill up, and the float needles rise into their seats to cut off the flow of gas, the pump senses this rise in pressure in the fuel line and stops momentarily starting up again when the pressure in the line drops.
The pump is often connected to the engine's ignition system so that it doesn't pump unless the engine is actually turning over. This prevents the pump from working in case the bike falls over and the engine dies but the ignition is still on. In many Hondas, for example, the pump is connected to a fuel cut-off relay that is in turn connected to the ignition control module. The relay prevents battery current from getting to the pump unless the ignition module is transmitting ignition pulses to the relay indicating the engine is running. No ignition pulses, no current to the pump.
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