Originally Posted by Zeke
What is the difference between on and primary then?
It's not primary
, it's prime.
In run the petcock is closed until vacuum from the engine (during and after starting) opens the petcock and lets fuel into the carb. When you switch off, the valve shuts and cuts off the fuel. Prime
allows fuel to flow all the time, and is used (for example) when the bike's been stood for a long time, and the carb bowl is empty. Then, you'd start in prime and switch to run when the engine's running. If you leave it in prime, fuel will keep flowing while the bike is stood, and fuel could leak into the engine, and possibly contaminate the oil.
Another thing... Why would you possibly want to ride in reserve all the time? Just doesn't seem to make sense.