Because the stock petcock is vacuum operated,.. it only allows fuel flow when the engine is running...
In the on position, it will run the fuel 'till it can't get anymore, and your bike will still run a bit, as it uses what's in the floatbowl...
Now, the floatbowl is empty, and yer' bike won't start, and without a running engine, the petcock won't flow fuel...
Sooo,... that's where Prime position comes in... it bypasses the vacuum circuit,.. and lets fuel flow down to fill the floatbowl... and then the bike will start...
Having a reserve position on a vacuum petcock, is basically a stupid thing...
so,.. yes... if you run out of gas, switch to Prime...
Sorry to ramble...

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