Another Aux Tank Thread

I'd read through several of the threads but being very inexperienced with fuel flow/management, I feel best I post some specifics of my situation to ensure I'm doing the right things.

I have a 1998 Kawasaki 1500 classic. it's carbed and has a fuel pump as noted in photos attached.

IF I intended to use this bike long term as a long hauler, I might approach things differently, but as I would prefer to end up with different LDR mount at some point (perhaps GW, BMW, FJR or Concours 14), I'm looking to do this in a more frugal manner. I don't want to be cheap at my own expense though.

I have been reading about using the vent line of the tank to feed fuel from the aux tank. my initial plan in October is to do a BBG. I've done a number of SS1k's. The BBG is my "test run" to do a CC50 in the spring.

I plan to mount the aux tank on a rack behind the rear seat and should be able to gravity feed. I'm thinking something about 5 gallons to be appropriate.

given my current circumstances, could anyone recommend a good tank to use, and how to plumb the fuel so as to be effective for my needs?

Thanks in advance for any thoughts! Resized_20210712_121328.jpeg Resized_20210712_121134.jpeg
On this bike there are two fuel shutoff valves fed into a T with the output going to the carbs. As long as both tanks are not on simultaneously on till one tank is empty, the floats valves will not be overcome by fuel pressure. The Aux tank is higher than the stock but not enough to cause an issue.


both tanks plumbed 04 29 17.jpg

How I typically run is start on the Aux tank run till empty turn on main tank.

so you keep your petcock for the main tank OFF until it sputters and then turn it to run....then when you it sputters again, you turn to reserve?

Russ Black

Active Member
It appears that your OEM fuel tank has a "Shoot Out" vent meaning even though it exits the tank on the bottom, the tube inside of the tank extends up to the top of the tank allowing it to vent. This is not a problem. This is how I have the Aux Tank on my girlfriends bike hooked up. I put a valve between the two just in case I need to remove the Aux tank with fuel in it or something goes arye. The vent allows air into the tank preventing a vacuum from forming when the fuel is pumped out. This is much like the vented vs non-vented gas can you use to fuel up you lawnmower. It flows at first, slows to a stop until the vacuum forms. Then it sucks some air back into the can and repeats. By plumbing the Aux tank to the OEM vent you have essentually the same thing. As the fuel is pumped out of the OEM tank via gravity or the fuel pump, it is replaced ounce for ounce from the Aux tank as long as the bottom of the Aux tank is above the top of the OEM tank. In essence the Aux tank will empty first and then the OEM tank. If you have a fuel gauge, it would read full until the Aux tank is empty and you only have the fuel in the OEM tank remaining.

If you haven't driven enough miles to empty the Aux tank, you can just fill it. If you have driven far enough to empty the Aux tank then you will need to fill both, starting with the OEM (Lower) tank and then fill the Aux (Upper) tank. There is normally no need to shut the valve off between the two tanks even when filling them, since the fuel line between the two tanks is small and not under pressure, the flow of fuel is slow as well. Fill and seal the lower tank and then fill the upper tank.

If the station where you are filling up has those uncooperative pumps that makes it extremely difficult to top off, you can achieve the top off by leaving the OEM cap off and observing it's fuel level as you put fuel into the Aux tank. The fuel from the Aux tank will slowly flow into the OEM tank and once the OEM tank reaches the desired level you can either put the cap back on the OEM tank or shut the valve until you can put the cap back on the OEM tank. Then finish filling the Aux tank as usual.

The only problem you might run into is if your fuel cap does not seal. Then you will need to shut the valve between the two tanks and only open it once the OEM tank is empty (or near empty) to allow the fuel to flow from the Aux tank onto the OEM tank. If your Aux tank is of a higher capacity than your OEM tank, you would monitor your fuel gauge and be sure to shut the valve off before you over fill the OEM tank. Without one I'd drain the OEM tank to the level where I'd need to switch the petcock to the reserve position and then time how long it takes for the OEM tank to become full. That would be the point that I'd shut the valve between the two tanks. Since you'd be riding while this is occurring, there should be no chance of overflow as long as you don't exceed that amount of time because you'll be consuming fuel as well. Then do it again when you hit reserve to drain the Aux tank and depending on how much fuel you get from the last fuel transfer session, either start locating a refuel point or run until you hit reserve again, flip the selector and then look for fuel.

Test everything for leaks. Test different fueling scenarios to see how to do them. i.e. 1/2 full Aux-full OEW, Full Aux-Empty OEM etc. That way there shouldn't be any surprises and you'll build confidence in your setup. Again, the fuel will be flowing slowly between the two tanks as the ID (Inside Diameter) of the fuel lines are small and not under any pressure than gravity. Let us know what setup you decided to go with. Good luck!
"so you keep your petcock for the main tank OFF until it sputters and then turn it to run....then when you it sputters again, you turn to reserve?"

Yes, but after running a few tanks you will know your range.