"off, prime, and reserve. no off" HuH?
on, prime and reserve???
Generally the petcock does not regulate the flow. It is either on (from main tank or reserve) or off.
The off postition is used mainly for parking the bike so you don't lose all the fuel if the carb develops a problem.
On your bike the Prime setting adds more gas to the carb for cold starting. most peds just use a choke and decrease airflow to increase the fuel air mix.