Fuel tap question

[LotusSevenMan]

Having a problem with the VTR running on one cylinder (front) but only occasionally. So chase fuel problem perhaps?
Started at the filter in the tank. Mine won't come out of the hole as I think the nylon has expanded due to 12 tears worth of sitting in petrol. Anyway, it's all clear. Now to the tap. Yep, all fine, except it isn't!
The bl%dy huge pipeways are clear and fuel flows through at a ruinous rate. The question is what does the vacuum diapragm do? Why do I ask? Well mine has a spring loaded diapragm which is fine, but in the vacuum elbow (and this item is mounted from the inside) there is what appears to be a one way valve. So, from new the rubber 'stop' valve is in place and holds back the fuel. The first vacuum that is applied by turning the engine over pulls it open and there it will stay. There is no 'calibrated bleed back' etc to allow the diapragm to relax and the internally fitted spring to again shut off the fuel. If this is the case then its only use is at initial fitment with some fuel in the tank and then only if you 'crack' the four screws holding the diapragm in place to get the air behind it!!!
Anyone have a similar tale/experience/thoughts?
Cheers, LSM

[Phil-VTwin]

I did strip mine a few months ago now as it was stuck due to not being used for 6 years.
I didn't pay much attention to it, just cleaned it out and removed all the sticky residue which was holding the diaphragm in it closed position but once clean it did work as expected, vacuum supplied to the pipe i.e. me sucking on it, opened the valve and when i stopped it closed.
From what i can remember it is the little air hole at the bottom of the tap which lets the air back in behind the diaphragm to close it.
I dont shut the the tap off if i am removing the tank, just disconnect the pipes and nothing flows through until vacuum is applied.

HTH

[LotusSevenMan]

Thanks for the reply Phil. That lower bleed hole is to allow fuel that might get behind the rubber shut off valve to drain away but the diapragm that is connected to it doesn't have that bleed as I said before. Very tempted then to poke out that internal rubber one way valve thing so it works as I would have expected it to do!!! If I don't shut the valve/fuel tap off I'll have a flood of fuel on my hands/bike etc.

[stormingjoe]

it delays the release of vacuum when you open wide, if this didnt happen the petcock would shut down and no fuel would be delivered.

[warby221]

it delays the release of vacuum when you open wide, if this didnt happen the petcock would shut down and no fuel would be delivered.

+2 on that
There’s very little vacuum when the carbs are full open

[LotusSevenMan]

All sounds very reasonable, but how quickly should the valve shut off when no vacuum/suction is applied to the pipe? Mine say happily for twenty minutes with no movement at all until I disturbed the screws to let air into the back and let the spring do its work. I might add that mine isn't sticking, or jamming. Just a good seal from the diapragm. I'm no wiser now so ????

[warby221]

Should be a bit quicker than that m8
When I’ve drained my tank in the past there’s only been a second or two’s delay from letting of the vacuum and the flow stopping

[Stratman]

You can get a tap/diaphragm rebuild kit for about £25 IIRC

[gilson]

Or convert it to a free flowing system...

To convert the tap you need to do the following:- (from Roger Ditchfield and advice for my track bike that now has a fuel pump fitted).

1) Remove the tap from the Tank
2) Split it into two halves by undoing the screws.
3) Remove the rubber diaphram
4) Cut out the middle so you leave the outer remaining "ring" to act as a gasket
5) Re-assemble the tap with the "gasket ring" in place
6) Blank off with Areldite or similar product the small opening where the manifold suction pipe fitted.
7) Blank off the Manifold suction pipe with a small bolt and wire it in place. It is better that you blank off the tube rather than the manifold take off itself, This way it can easily be returned to Standard.
Re-install the tap in the tank
Job done

Mind you there must be a reason why there's a vacuum on there....

[LotusSevenMan]

Well checked it again today while doing other work. Haven't run the bike in a week. Pulled the fuel pie off....................... fuel pi$$es everywhere.
So I guess I have a Roger modified type tap without any mods at all. Grrrrrrrrrr

[IgorVigor]

5) Re-assemble the tap with the "gasket ring" in place
6) Blank off with Areldite or similar product the small opening where the manifold suction pipe fitted.
7) Blank off the Manifold suction pipe with a small bolt and wire it in place. It is better that you blank off the tube rather than the manifold take off itself, This way it can easily be returned to Standard.

Sorry to drag up an old topic, but wouldnt the easier solution rather than trying to araldite the breather and blocking the vacuum tube, to run a short length of tube between the breather and the vacuum point?

then just block the pipe from the inlet manifold (or connect it to a scottoiler or the other inlet, with a nipple)