Milo...
Don't continue to crank the starter motor until the battery is low! If the engine doesn't start after a couple of seconds, find the problem, or you will have two problems: your original problem AND a dead starter motor!
Next time you get the problem:
1: Stop cranking immediately
2: Turn the petcock to "prime"
3: Wait 5 seconds
4: Try to start the engine
If it starts immediately, there is a problem with either the vacuum shutoff valve in the petcock, or in the vacuum pipe that connects to it.
If the engine still does not start, there is another problem.
Tell us what happens, and we can make more suggestions.
Lastly:
Do NOT Forget to turn the valve back to "On". Don't leave it in the "Prime" position.
Explanation (in case you don't know this; forgive me if you do):
There is a gasoline valve which is part of the petcock inside the fuel tank. That valve is operated by a vacuum (technically, only a low pressure - a mild suction) created by one of the carburetors. The purpose is to shut off the flow of gasoline when the engine stops. It takes only a second of cranking for the vacuum to develop and operate the valve to let gasoline through.
If the vacuum valve sticks closed, or if there is no vacuum fed to the valve (a leak/break in the pipe, or a disconnected pipe), then the valve won't open.
The "prime" position allows fuel to flow without operating the vacuum valve.
