How to refill the reserve gas tank 1981 Honda CX500 motorcycle

There is only one tank to fill on your bike, your getting confused by the reserve. The reserve is just on the fuel tap itself. It has three positions, on , off and reserve. In the on position your fuel tap will be able to drink about 90% of your fuel tank and then it wont be able to drain anymore, then you must turn your fuel tap to the reserve position and it will be able to drink rest of your tank approx 10% that is left. So next time you fill up at a petrol station make sure your fuel tap is set to on, then when your bike starts to splutter out of fuel you can turn your fuel tap to reserve and get yourself to the petrol station to fill up, making sure you then turn your tap to on again. If you keep running out on the reserve it can cause debris in your tank to clog your carb up.
hope this helps
steve

The petcock valve is what controls what is called the reserve tank. As mentioned above, there is only one tank. What the valve does is prevents the fuel from draining all the way to the bottom of the tank. When you turn the valve to reserve it allows the fuel to drain all the way tothe bottom.