Are overflow tanks necessary on aux fuel?

Shawn K

Professional Cat Confuser
Premier Member
#21
One thing with AUX tank, don't put a line in cap to it lower than bike, when it heat up and start draining it will empty you tank and all tanks if it connected to rest.
Had 8 gallon with extra tanks, drained 2-3 gallon out before some called me and said FIRE HAZARD.
My new bike i got separate 2 gallon (5 gallon total) with valve and fill it almost all up no issue with overflowing leaving about 0,5 - 1 inch below cap.
Is a check valve in the line sufficient to keep that from happening?
 

EricV

Premier Member
IBR Finisher
#22
I think AtoZ is talking about a siphon effect on the vent line. Normally that wouldn't be possible if you had a valve between the main and aux tanks. Air has to come in to allow fuel out. W/o the main tank's vent to do that, you wouldn't get the siphon effect.

And with a tail dragger where you're pumping fuel when you need it, again, not an issue. This is also why you want an air gap at the top of the tank for expansion so you don't puke fuel before you need it on a hot day.
 

keithu

Premier Member
IBA Member
IBR Finisher
#23
I think AtoZ is talking about a siphon effect on the vent line. Normally that wouldn't be possible if you had a valve between the main and aux tanks.
Another issue to be aware of is if your bike has a charcoal filter on the main tank vent. This was common on California model bikes for many years, and is now also showing up on some Euro5-compliant bikes. The filter is supposed to block evaporative emissions. If you overfill the main tank the evap filter media can become soaked with liquid fuel and then it may no longer vent properly.

On my Ducati, the soaked evap filter would vent in but not out. So on a hot day fuel vapor pressurized the main tank. When I opened the aux fuel ball valve this pressure pumped the remaining fuel back to the aux tank and out its vent.

If you hear "PSSHHH" from escaping pressure when you open your fuel filler on a warm day, you have a venting problem like this. It's usually not difficult to perform an evapectomy and convert the bike to a 49-state vent.