Lack of petrol flowing
 

Out at Oulton yesterday and a previous afliction reared its head. After a few laps (5/6) the old girl ran out of puff at turn 2. As they had limited marshalls I tried to limp home and after knickerbrok she suddenly had a second wind!

Second session and the same happened again :confused:


Now I had an issue at Snetterton last year thta felt like a fuel issue, I had the carbs stripped and rebuilt and set up for the new pipes in time for Cadwell and it were fine at the end of last year. Now the Mrs did drain my tank to the bottom for the petrol!

Got the bike out today, started to drain the tank and what started as a good flow turned into a trickle. Then after unscrewing the petrol cap (replacement race item) the flow was restored!

I suspect either
1. there is rubbish in the float bowl again
or
2. The flow is not enough

Its a monster tank with the vacuum pump (which I thought we bypassed but I may be wrong). How does the air get back in the tank to replace the fuel going out of the tank so we dont get a vacuum? There seems to be no hole in the race fuel cap although there looks like a mini ball bearing (0.5 to 1mmm in size). The SS tanks used to have a pipe on the top with a valve? What does the monster get?

Am I looking at the right thing or am I going to have to pull the carbs kff to look at the float bowl.

Tried blowing through the fuel cap to see how air could get in and it did not seem to work.

Very puzzled bloke
:confused:

Hi Mark,
I do believe it may be your tank not being able to breath, remove the filler dap and remove the ball bearing, replace and try again, this is what happened when I put a Chinese one on my 748.


Chris:burn:

Having chatted to local mechanic he too suspects the lack of the tank to breathe. Cant see any way to remove the ball bearings but will have a good look.

Cut open and inspected the fuel filter. Learnt its ok to blow through the fuel filter but sucking is a silly thing to do.

Little bit of rubbish on the tank side of the filter (need new filter now I have cut it open.)

Tried blowing past the ball bearing from both sides but to no avail. Definatelt lack of air. Get a good old hiss when I open the cap. I suppose drilling out the ball bearing is not acceptable on a race bike.

Cheers
Mark

Drilled mine out, so do not see why you cant.

Chris:burn:

I drilled a hole in the filler cap on my race bike.

Never failed scrutineering or let out too much petrol during a race!

Woo Hoo,
out with the drill then. Cheers Chris and Neal

Mark :D

Although two scrutineers now know about it :devil:

Spoilsport:frog:


Chris:burn:

Maybe buying a new filler cap unit may be needed to keep the scrutes happy;)


Chris:burn:

And one of them is using it this year I believe?

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

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

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

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

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!

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...

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

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.

Cap ordered!
Couple of fuel filters ordered.

Just hope it all works

:)

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

1. Not sure where the outlet is!

I cant see a valve so I am sure there must be some need for this. I was assuming one of the two outlets at the back of the tank were something like this. Might try turning the tank around a few angles to see it owt leeks and then figure if it leeks it needs something.
Cheers
Mark

From memory its out the back of the tank at the bottom near the fuel filter, one long pipe which goes down thru the frame towards the bellypan

Mark I thought I would put my answer to your PM here as others may want to comment or take note

"Yes the Breather comes through from the underside of the tank, the other is the drain to stop rain water building up and then getting into your fuel every time you open the cap

On the underside of the tank on my 750 Monster they are marked Bleed and Drainage respectively
These can block up and I have had to use welding wire to clean them out before
The Drain is outside of the fuel cap seal so under normal circumstances shouldn't see petrol other than when over filling or being a bit sloppy when filling . The bleed should come up to a small rubber grommet like thingy that lines up with the breather in the standard filler cap which should contain a one way valve so no catch bottle needed

Most cheap Aftermarket filler caps fitted to Ducatis are actually Yamaha ones and the breather is in the wrong place to align with the bleed on the Ducati tank.

New fuel cap on. Countersunk bolts on order. The new cap mechanism needs countersunk, where as the old one did not. Try getting bolts on a Saturday!

All on and good flow with a new fuel filter added as I cut the last one open to have a look see.

Have revelaed most of the frame number, enough to see it hsa not been tampered with. Hope thats sufficient. I am afraid to rub much more it looks like it could be rubbed away.

Many Thanks folks

:)