Petcock rebuild

Well I rode the mighty yamaha today and notice a little fuel leak from the back side of the petcock. So, I removed everything and even found a step by step rebuild guide on in the forums. I put everything back together and noticed that the directions point out that the rubber gaskets go on either side of the plastic middle piece. Is this possible ? I say that because the diagram has the two gasket on either side on there is no way to remove them with out tearing them to put them in between the plastic part. I have a service manual and it only covers the front part of the petcock rebuild.

 

There is no such thing as a stupid question. Dont ever forget that. Ever.

This got me as well the first time I rebuilt my petcock. I stood there for like 15 minutes... I pulled at the one rubber diaphragm. It didnt come off. I tried to "unscrew" the plunger into 2 pieces. It didnt unscrew.

Then it hit me. I wadded up the one diaphragm around the O-Ring side of the plunger and crammed it through the hole.

POOF! My mind was blown.

 

GENTLY wad the diaphragm up and tuck it through the hole.

 

I got it all back together a few days ago after watching a random youtube video. Now, it runs best only on reserve (fuel flows with no vacuum). I think I screwed something up.

 

The last vacuum tap I rebuilt ('93 GSX-R 750), had a small hole manufactured into the diaphragm, it lined up with another hole in one of the castings and another in the plastic plate between the two halves of the diaphragm. It lets air in between the two halves of the diaphragm, letting the vacuum pull the diaphragm back against the spring and letting the fuel flow. XJ's are slightly different; I found a thread from our resident bigfitz52 whilst trying to find a picture describing what I'm trying to explain:

Bigfitz52's thread

That explains the entire rebuild process in quite a bit of detail.

There are no holes in the diaphragm itself, the plastic plate handles the air passage. I would check that yours is the right way around, but looking at the castings on the petcock body itself it should be near impossible to get wrong.

If that's right, then make sure the spring between the diaphragm and the petcock body is seated properly, otherwise it won't be able to shut off the fuel when there's no vacuum.

 

I was looking for that thread via the search feature, and didn't find it. Perhaps I take her back apart tomorrow.

 

Sometimes it's easier to search the forum through Google, if you use "site:xjbikes.com " (minus the quote marks) and then your search. You should only get results from this site.

 

Truth be told, there are quite a few ways to get it wrong. The plastic plate has a "front" and a "back" and only goes in one way.

Furthermore, the little vent passage needs to be properly oriented.

And don't overlook the little bakelite disc "check valve" located in the output flange behind the hose nipple. Not only does it need to be there and in good shape, it needs to be free and able to sit flat in its housing in order to work properly.

Plus the seat for the diaphragm valve needs to be in good shape.

Remember, not every petcock can be rebuilt. They were made from some pretty cheesy metal to begin with; and it's possible for them to deteriorate or wear to the point of being un-rebuildable. Fortunately, only about one in four are that bad, so it's worth it to take a crapshoot on eBay for $5 a pop.