Hi, I have a 2005 Multistrada 1000DS which gave me no end of grief due to a faulty roll-over valve. Due to troubleshooting this I gained a good knowledge of the ins and outs of the fuel tank system, so if you are having a problem with filling your fuel tank, here is an explanation to help you understand how the tank breather/overflow/filling units work:
At the petrol cap you can see 3 relevant components:
1. The overflow hole – in the recess
2. The rubber grommet – which is a breather hole
3. A tiny hole in the cap that the grommet seals around when the cap is closed
Under the pillion seat you will see 2 relevant components:
4. The fluid recovery reservoir (black box)
5. The reservoir breather valve (the white bubble) – also known as the roll-over valve
So once familiar with those components find the overflow tube that hangs down on the left hand side of the bike (next to the oil cooler hoses). Follow that up and it branches out into 2 tubes, each ending at a metal insert into the tank - one on either side. These do not lead directly into the tank but into tubes inside the tank - the one on the right hand side leads to the overflow hole ( #1); the one on the left splits inside into 2 – one leads to the rubber grommet (#2), the other to the fluid recovery reservoir (#4).
It is important to know that the shape of this tank is far from “normal”. To get 20L packed economically into the design, Ducati created a U shaped tank, with the bottom part of the U running under the seat. Looking at this from the left hand side, the high point of the left hand side (LHS) of the U is the filler cap, that of the right hand side (RHS) is the top of reservoir breather valve (#5).
An explanation of these components:
Function of #1: When it rains and you get water around the edges of the filler cap or the fuel in the tank expands or if you over-fill the tank....the liquid runs down into the overflow hole and drains out onto the road – via the overflow tube.
Function of #2 & #3: these allow air into the LHS tank with the fuel cap closed: as the fuel in the tank drops the air can enter the top of the tank – being sucked up through the overflow tube that #2 connects to and into the tank via #3.
Function of #4: It looks like this was added to meet EPA requirements – it is designed to capture any fuel and gases that may escape from the tank (due to the valve not closing quickly enough?) and these will be drawn back into the tank due to the vacuum created when air is being drawn back in. If interested read more on this (and the other components) in the patent application: http://www.freepatentsonline.com/EP1466823.html
Function of #5: This allows air in and out of the RHS tank. It has a spring-based float with a shut-off valve, so that when the tank is being filled the valve is open and allows air out of the tank (into the fluid recovery reservoir -#4) until the petrol rises to the top forcing the float up to seal the valve. Then as the level of petrol drops the float drops and the valve is then open to allow air back into the tank to replace the petrol. The air is drawn back into the tank through the fluid recovery reservoir (#4) which sucks it in from the overflow tube, via the tank’s inner tubing.