i dont have a motobecanebut this sounds like a fuel tank valve, properly known as a 'petcock'.
Off turns the fuel off at the tank.
"On" allows the tank to drain down to the 'RES' level. (there is still some fuel in the tank but the bike will run out of gas at this point)
The "res' is Reserve. There is probably about 20 ounces of fuel in the tank accessible when 'RES' is selected. When you run out of gas with the "ON" setting, switch to RES and restart the bike.
To check/see if this petcock and it's slow fuel flow is responsible for the slow road speed, remove the fuel line from the carb and drain some fuel into a container . A steady stream of fuel should flow at both the ON and RES settings.
If not, it's a good idea to clean the tank, cap (make sure it vents), fuel lines ... may as well clean that carburetor too and get that entire fuel pathways working properly..
Get yourself a manual ASAP and learn about your bike... might be one available in the "resources" up there at ths top of this page..