There's been a lot of talk lately about fuel problems, and well lets face it it's gonna happen. And in this thread I'm gonna discuss one of those things related to fuel and that's the fuel petcock. When it comes to fuel issues a lot of people sometimes dismiss the fuel petcock as a potential source to their problem. The fuel petcock is a vacumm operated system which works in conjunction with the lower pressure fuel pump. Here's a pic of the fuel petcock:
You'll note that the fuel petcock has 3 settings, On, Pri, Res. In the ( On ) position fuel will only flow if the engine is running. This is because the fuel petcock has a diaphragm in it which prevents fuel from flowing when the engine isn't running. This diaphragm can also cause poor running conditions if it dries out or goes bad. In most cases it is noticeable by poor idling, stalling, etc. Though in some cases it can sometimes lead to fuel flowing when the engine isn't running, thus allowing fuel to enter the crankcase and contaminate the oil. If you suspect the fuel petcock is at fault pull the vacumm line off the petcock and if you have fuel in the line or fuel comes out the petcock where you removed the line then it's bad.
When set in the ( Pri ) position fuel always flows regardless of if the engine is running or not. This is because when set in ( Pri ) position fuel bypasses the diaphragm and runs straight to the carburetor. This is a good test to do if you suspect the fuel pump isn't working. Note:
If you do this return the valve on the petcock back to the ( On ) position when the bike isn't running, failure to do so may result in fuel entering the crankcase and contaminating the oil.
And when set in the ( Res ) position fuel is being drawn from your fuel reserve. This should only be used if you run out fuel and need to use your reserve.