Aux tank larger than main tank

keithu

Premier Member
#1
I have a Yamaha FJ-09 with a stock tank capacity of 4.8 gallons. I want to install a ~5 gallon aux tank above the pillion seat, plumbed into the main tank with a bulkhead fitting.

My concern is about overfilling the main tank and spilling fuel out the vent. When the fuel warning triggers on the FJ I can usually put 3.8-4.0 gallons in the tank. So my thought is to open the aux tank petcock only after the fuel warning triggers. The bottom gallon or so of fuel in the aux tank should be below the top of the main tank, which should prevent spillage.

Thoughts? Am I missing anything?
 

Marc11

Premier Member
#2
Keep in mind you'll be burning fuel as you transfer so, depending on transfer rate it may not be an issue. Me, I'd either not fill the aux tank to capacity or keep an eye on the fuel gauge and turn off the tap when it reaches full.
 

Stephen!

Premier Member
IBA Member
#3
Were it me, I really don't think I would willingly install an aux tank larger than my main tank. Only bad things can happen when you are fatigued. With the K16, over filling the main tank dumps raw fuel directly into the intake. I learned this at zero-dark-thirty in the middle of Nevada when I forgot to close the valve before refueling. The bike runs really bad in this configuration. :eek:

If I was somehow forced to accept a larger aux tank then I'd adopt a new fueling strategy. Something along the lines of a system where I would dump the "excess" fuel into the main tank once I knew there was enough room, then close the valve until the low fuel light came on... Knowing where the fuel gauge indicates enough room for the excess fuel is, of course, critical to a system like this. As is figuring out how long it typically takes to transfer that much fuel then having some way to remind you to close the valve if you forget to monitor your gauge. My GPS has persistent "User" timers that could be set for this function.

A system like this is fraught with perils, especially when you are fatigued.
 
#4
The only issue I had with an auxiliary fuel tank was from letting the bike sit in the hot sun after filling both tanks. Gasoline would spill out of the auxiliary tank.

My routine was to fill both tanks, then drive away and transfer some fuel every hour or so. My auxiliary tank was smaller than the OEM tank. (Gold Wing)
 

keithu

Premier Member
#5
Keep in mind you'll be burning fuel as you transfer so, depending on transfer rate it may not be an issue. Me, I'd either not fill the aux tank to capacity or keep an eye on the fuel gauge and turn off the tap when it reaches full.
Unfortunately the FJ-09 fuel gauge shows "Full" at any level above halfway. So in theory when I reach the bottom level on the guage I could open the petcock for a little while until the guage shows Full and then close the valve again. In practice that sounds like something I'd screw up after a long day.
 

keithu

Premier Member
#6
A system like this is fraught with perils, especially when you are fatigued.
Yep, my concern exactly. My last bike with aux fuel was a Triumph TT600. It had a 4.5 gallon aux tank, and I installed just a 3 gallon Summit cell. When the low fuel light came on I could open the aux petcock and just not worry about it. Perhaps I need to just get a 4 gallon cell for the FJ. I'm sure 8.8 gallons would meet my needs... But wouldn't 9.8 gallons be even better?
 

rneal55555

Premier Member
#7
Yep, my concern exactly. My last bike with aux fuel was a Triumph TT600. It had a 4.5 gallon aux tank, and I installed just a 3 gallon Summit cell. When the low fuel light came on I could open the aux petcock and just not worry about it. Perhaps I need to just get a 4 gallon cell for the FJ. I'm sure 8.8 gallons would meet my needs... But wouldn't 9.8 gallons be even better?
Sometimes less is more. I understand your thinking but id you really need the extra fuel why not use two 2.5 or 3 gallon tanks you could them dump them one at a time when you get the low fuel light with no worries about overflowing.
 
#8
Did a little research on the Yamaha FJ-09 but could not see if the fuel pump as internal to the tank. If it is not you can T in the supply side. Turn the main tank off and run the aux tank till empty then turn on the main tank.
 
#9
I had a R1100 GS 25 ltr main tank with a 16 ltr aux tank in place of the pillion seat gravity fed into a spur lower point of the left lobe of the main tank, fill both and ride with the petcock open liquid will find it's own level as the fuel was used from the main it topped up from the aux as there was less weight trying to feed it did not over fill
DO NOT have a aux tank bigger than your main it will flood the main tank if you leave the petcock on by mistake easy to do when you are tired.
 

SteveAikens

Premier Member
IBR Finisher
#10
There is no way I would even consider a cell larger than the stock tank on a motorcycle. The possibility of failure simply being there is enough for me.

Understand, I very good at fuel management - and I would not try this. The old saying "Crap occurs" should be enough of a warning - all in my humble opinion of course...;)

The very thought of the possibility of a scrotum soaked in gas should be a deterrent.
 

keithu

Premier Member
#11
Did a little research on the Yamaha FJ-09 but could not see if the fuel pump as internal to the tank.
The pump is inside the main tank. Also, the fuel supply is returnless so there is no return line I can T into. Hence my plan is to use a bulkhead fitting. No biggie, I did it that way on my Triumph.

Thanks for the feedback everyone. I think the safest thing to do will be to just install a four gallon aux tank. The idea to use two 3-gallon cells is interesting, but I can easily think of a few ways for that to go wrong.
 
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Stephen!

Premier Member
IBA Member
#12
But wouldn't 9.8 gallons be even better?
The only time you have too much fuel is when you are on fire... :eek: (...or if it puts you over gross in an aircraft)

I suppose if one was up to the challenge, one could install a float switch (like for a sump pump) in the main tank and connect it to a solenoid controlled electric valve that drains the aux until the float switch turns off the solenoid....

...or maybe use the float to mechanically open and close a foot valve installed inside the main tank...

...or any other number of overly complicated systems... :p
 

Marc11

Premier Member
#13
Me, if I already owned the tank and was worries about the mental mistakes I'd only fill the aux tank to just about a gallon under the main tank capacity and carry on.
 

cacomly

Premier Member
IBR Finisher
#18
The idea to use two 3-gallon cells is interesting, but I can easily think of a few ways for that to go wrong.
Alternatively you could have a tank built with a partition and two outlets. I'm was considering doing something like that on my 2007 Sportster which is fuel injected.
 

Marc11

Premier Member
#19
That's an interesting idea. You could also install a petcock like on carb bikes with a main, reserve and off position where the main pick up tube is higher than the reserve. Therefore the main drains say 4 gallons and the reserve setting the last gallon.

I like this idea the most...unless you screw up and turn to reserve from off in error.
 

Airportbum

Premier Member
#20
Measure the 5 gallon tank if you already bought it. My 5 gallon tour tank only holds 4.3 gallons, 4.4 on a super cold day!
If the deal is better on a five gallon or you have already bought it, you could just fill it less than full on each fill up...then no overflow.