PRIME = ON
Reserve and off are what they are...
The only regulation of fuel flow is in the bowl. That's where the float comes into play.
The regulation of fuel going into the motor is dependent on, A. Size of the main jet in the carburator. B. Size of the carburator itself. C. The size of the inside diameter of the intake manifold.
The petcock is there because it's VERY BAD to store your moped with the fuel constantly ON. You could cause leakage in the carburator, and also corosion can get into the carburator due to condensation developing if left for a long period of time.
When storing your bike or leaving it over night in a hot/cold climate, turn the petcock OFF and let it run out of gas on it's own. Then in the morning when you go to kick it over, turn the petcock to PRIME or ON and flick the choke ON. It should start right up in a couple of cranks.