Ducati Sporting Club UK > DesmoDue Race Series > DesmoDue - General Questions and Chat > Lack of petrol flowing

[ells]

If it needs to be done for scruitineering then thats good enough for me!

Looks like its off to flea bay for me. Unless anyone else knows of a good source?

Mark

[skidlids]

Shame you can't find someone with a SS that would do a swap as the SS has a seperate breather at the front of the tank

[ells]

bit of a thicko question but when the petrol leaves the petrol tank how does the air get in to replace it?

There are two pipes leaving the monster tank at the back base of the tank? Do either of these supply the air?

Cant see that even if these two ball bearings in the cap mount were working where the air would come and go to easily!

There are two holes in the top of the tank when you take the whole machanism out! Do they supply air? More complicated than I firsat thought. Suspect if I drilled the mount rather than the cap then this may not makle any difference!

Very confused
Mark

[bradders]

doesnt it come in via the cap? There are small pin holes in the std one which lets it in.

[antonye]

Originally Posted by ells

bit of a thicko question but when the petrol leaves the petrol tank how does the air get in to replace it?

Cant see that even if these two ball bearings in the cap mount were working where the air would come and go to easily!

The ball-bearings are (should be!) held in by a spring and a worm screw. It operates as a valve, allowing air to flow in to the tank.

The ball bearing should be pressed up against a hole by a spring. It's all held in place by the worm screw. What happens is that the pressure inside the tank builds up, it sucks the ball bearing away from the hole, effectively opening the valve, and allows air inside. Once the spring overcomes the pressure (the reduced pressure) it pushes the ball bearing against the hole and seals it again.

The worm screw is used to hold the spring and bearing in place and also to set the preload tension of the spring. It may be that the spring is wound in too tight so the valve will not open correctly ie, the pressure difference cannot overcome the spring to move the ball bearing and open the valve.

If the fault lies with the fuel cap then you may find that adjusting the valve is all that is needed. Drilling it out is not a good idea, it just needs setting up properly.

As for how the air gets in - the fuel cap itself is not air tight, so air can get in through the cap, either via a dedicated air passage in built into the cap or through the thread of the cap itself, depending on the design.

HTH!

[numbskull]

Perhaps if Kev gets a minute, he could post a picture of the holes I drilled in the top of my GSXR filler cap in my ali tank from last year...

[ells]

Think that helps with the fuel cap.

So why do I have no catch bottle for petrol overflow? Surely there must be an overflow pipe for the petrol! Theres still two pipes at the back of the monster tank, are they both overflows for the tank? Cant see a good reason for two pipes.

Thanks a lot
Mark

[antonye]

Originally Posted by ells

So why do I have no catch bottle for petrol overflow? Surely there must be an overflow pipe for the petrol! Theres still two pipes at the back of the monster tank, are they both overflows for the tank? Cant see a good reason for two pipes.

On the fuel injected systems they use a pressure valve at the throttle bodies, which is set to around 3 bar, I think.

If the pressure builds up more than this, the valve opens and the fuel flows through the valve, straight back into the tank. It's effectively just pumping the fuel round in a circle, rather than into the throttle bodies. This is a closed-circuit system, hence it doesn't need an external vent/overflow. Again these are usually nothing more than something covering a hole (like a blanking plate or ball bearing) and a spring which has a specific tension, just like in the fuel cap, only they work the other way round in that the pressure is pushing against the spring (so the increased fuel pressure overcomes the force of the spring) rather than it pulling on the ball which acts against the spring.

Not sure how it works on the carby models tbh, but I'm sure that the carbs have an overflow pipe that probably vents to atmosphere by default if they do overflow. Normally the fuel would just shut off if it's a gravity-fed system because the pumps are much lower pressure.

[ells]

Cap ordered!
Couple of fuel filters ordered.

Just hope it all works

[bradders]

have you got a non return valve on the outlet Mark? I think Skids fitted one to Craigs bike last year so no need for a catch tank