Aux Fuel Tank for Cannonball
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2010 ThunderFly 190, 2008 250 GTS
Joined: 29 Apr 2011
Posts: 2653
Location: Springboro, OH
Fri Feb 14, 2020 11:33 am quote
As the title states, I'm looking to add an aux tank to my GTS for Cannonball. It needs to be easily installed and easily reversed. I've scoured older threads here on MV and elsewhere on the web, and hoping I can get definitive guidance from the experienced folks here.

I'm planning a simple gravity feed solution, with the tank on my rear rack. I plan to remove my top case for the SCR. I've found a 2.25 gallon (there is also a larger 3.5 gal) aux tank with vented cap and outlet for a vent tube as well.

I would like to be able to control the flow of gas from the Aux tank with an inline valve (like a reserve switch) and plumb to a 90 degree elbow fitted to a drilled gas cap on the main tank. Fuel would flow from the Aux tank to the Main tank through the modified cap. An inline quick-disconnect would allow me to disconnect the end with the modified cap to allow for refueling of the Main tank.
The vent tube from the Aux tank would connect to the Evap canister via T-connector in the existing evap line.

My primary concern: is this too simple a design? As the fuel level in the Main tank drops, will this impede the flow of fuel from the Aux tank?

Any suggestions would be appreciated.

Vespa Aux tank design.JPG

Veni, Vidi, Posti
2008 MP3 500, 2013 BV350, 2020 Vespa Sei Giorni
Joined: 13 Oct 2012
Posts: 7915
Location: Ashburn, Va. Home to the Internet
Fri Feb 14, 2020 11:58 am quote
Think you may have to leave the Aux vent open to environment.
Give it a try.
2016 Sprint S 150 (his), 2016 Sprint 150 Blu Gaiola (hers), 2006 GTS 250 Excalibur Gray (hers), 2006 GTS 250 Black (his), 2012 Kymco Downtown 200
Joined: 21 Oct 2016
Posts: 747
Location: Vermont
Fri Feb 14, 2020 12:30 pm quote
I am far from any expert but just a thought on pressures. The aux tank is metal, will be mounted out in the sun and getting warm. This would cause some expansion and pressure increase in tank, although small but since the fuel tank is plastic and not exposed, could the aux tank back pressure the main tank with a negative effect on fuel delivery?
Moto Giro Titan
2009 GTS 250 Super Lucrezia Borgia, 2013 Ducati Hyperstrada, Little Big Red
Joined: 04 Feb 2009
Posts: 2550
Location: Carrollton, Kentucky
Fri Feb 14, 2020 12:44 pm quote
Not sure if Bagel aka Tom Donahue is on this forum, but he has ridden to hell and gone more than once on his GTS, and he has an aux tank in the top case. It would be worthwhile to pick his brain.
LXV 150 3v ie. Midnight Blue (Sold)
Joined: 06 Dec 2010
Posts: 3131
Location: Bangkok
Fri Feb 14, 2020 9:11 pm quote
is this too simple a design?
Looks relatively complex to me. Fuel cannot transfer from the auxiliary tank unless it is vented so I would only have one valve for venting the aux. tank.
Ride scooter until fuel gauge shows xx% level or the fuel light blinks then simply open the vent and the fuel will flow into the main tank and then show on the fuel gauge. The rate of flow between the aux . and main tank is irrelevant as long as the fuel line is normal size.
If you have a aux. tank larger than the main then shut off the fuel when the main tank shows yy% or full.
GTS, LX, VSD, VSX, VNX, LD 125, Chucky, LI125
Joined: 21 Oct 2005
Posts: 4181
Location: San Jose CA
Sat Feb 15, 2020 9:35 pm quote
Did this in 2012, 5 gallons, kept the weight low with a custom mount, worked great.
The fuel was plumed *THRU* the left rear fender(The valve at the end of the yellow fuel line.) as shown in the first photo to the underside of the main fuel tank and both vent lines tied together.
7.4 gallons will take you a long long way.
Could be for sale...

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Joined: 29 Apr 2018
Posts: 733

Sun Feb 16, 2020 5:47 am quote
The way to do it. And it does not have to be completely full.
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