If you are new to an aux tank the following info will be helpful.
The system is entirely gravity feed. It is essentially a jerry can that is plumbed into the main tank. When the main tank gets low enough you open a valve on the aux tank to transfer the fuel from the aux to the main.
Here is the process I have been using for years that works well for me. This process has been used on BMW and Triumph.
- Wait until the low fuel light comes on.
- Open the aux tank valve.
- Monitor the low fuel light for it to go out. This gives you indication that you have fuel transfer to the main tank.
- Leave the aux tank valve open until you are ready for more fuel.
- Close the aux tank valve before refueling. This is important (see later).
Here are some caveats to be aware of:
- If the aux tank valve is open when the main tank is full, you can overfill the main tank. If you have a bike WITHOUT emission controls, the overflow will spill to the ground and you'll likely lose all of your fuel. If you have a bike WITH emission controls (most nowadays), the overflow will saturate the charcoal canister (part of the EVAP system) and cause the engine to run rough (rich) until the charcoal canister gets dried out. This can take a few minutes and will scare the snot out of you. I've only experienced this when the bike was parked. It may not happen if the bike is running but I've never experimented with this scenario.
- The main tank vent is designed to let air IN not OUT. Filling the main tank this way is doing something the main tank wasn't designed to do; that is, to let air out of the vent. As such, fuel transfer can be slower than you expect.
- The gas gauge will not respond quickly to the change in fuel level. It can sometimes be miles before the gauge will respond. That's why I monitor the low fuel light, it is much more responsive.
- On rare occasions fuel transfer does not initiate. This seems to be related to the aux tank not venting. This is especially true if you have the overflow tank. Sometimes the siphon doesn't get started. This is nothing to worry about but can be a distraction. I decided once to just let it go and see what happened. My low fuel light stayed on and the distance to empty stayed at 0 for a long, long time. So long that I finally decided that what was happening was that when the main tank got low, below the fitting, a bit of air would go into the aux and allow some fuel to transfer. This was happening enough that I wasn't running out of gas but wasn't getting good fuel flow. Eventually, there was finally enough draw to make things work properly. The key thing to remember here is that you DO have 4 gallons of fuel, so don't worry!
- If this bothers you (I know it does me) you can reach back and loosen the gas cap on the aux to start the flow. Don't forget to put it back on.
- Finally, the BIG note. DON'T FORGET that you have drained your aux tank. There have been a few times (in a fatigued state) that I have waited until the low fuel light came on, reached back to open the aux valve and discovered that it was already OPEN. Yikes! You'd better be close to some gas and not on this road...