Fuel Petcock Diaphragm

[Karl Hughes]

 

In the absence of the XJ group site (for the time being hopefully) I thought that I'd just pass this on for future reference and inclusion in the new group site.

 

I obtained a new rubber diaphragm for the fuel petcock some time ago and got around to swapping it in a couple of weeks ago. The old one was still OK but had all the signs of age last time I looked at it.

 

So, out with the old and in with the new and...fuel pouring out of the petcock like there was no tomorrow (and there wouldn't have been if someone dropped a cigarette).

 

Rick helpfully gave advice on using a sealant around the edges to ensure the leakproofness when assembling the parts so I opened it up again, placed gasket compund around the edges and put it all back together and...it still leaked.

 

Went onto the XS750 triple site over the weekend (no XJ site...) and lo and behold found the answer.

 

It appears that some aftermarket non-Yamaha diaphragms, which come with a rear plate on one end and a plunger like protrusion with an O ring around it at the other, may be 0.5mm too short! The plunger end fits into the body of the petcock and stops the petrol going through it until the vacuum from the engine activates the diaphragm thereby alternatively opening and closing the valve via the plungers movement. Also, it was recommended to stretch the small spring inside the petcock to ensure good pressure is applied the diaphragm when at rest. My spring when I initially extracted it...(no jokes here fellas)...was at best, flaccid.

 

I took the old used diaphragm assembly, cleaned it up and used the sealant around the edges to ensure there were no leaks. Put all the petcock bits back together and miracoil of miracoils...no leaks. Petcock works perfectly and doesn't leak when the fuel hose is disconnected.

 

I would still like to replace the old diaphragm and may have a closer look to see if the one I purchased can be moved on its shaft a bit to be of the correct length so that it won't cause a leak.

 

However, if anyone has purchased a petcock diaphragm it would be appreciated if you could advise from where...old bike barn or some place similar?

 

I feel like I've just constructed a post full of sexual metaphors...

 

Cheers folks,

 

Karl