[JaVeRo]

If you can't switch to the auxillary tank then it is most likely the solenoid valve underneath the frame. The right (passenger) tank is the primary tank and the left is the auxillary. Look for fuel lines coming from each tank and they come into a valve that, with no 12v current, allows fuel to flow from the right tank and with current, draws fuel from the left tank.

In a pinch you can swap the lines and make it draw from the left tank with a bad valve or no 12v current going to the valve.

If there are 3 ports, 2 in and 1 out, you are in luck. That is the cheap valve.
I believe the newer ones have 6 ports, 3 for the return lines. I have never changed one of the later ones but have heard they are more expensive.

James