Rebuilt petcock still not working

I have rebuilt a number of XJ fuel valves using Fitz's procedure, but this time I'm not having good luck. The petcock is from an '81 XJ650. It does not leak fuel externally or past the valve, but it just isn't operating properly when in the "on" position.

When I blow and suck on the vacuum nipple it seems like the little disk is moving as I would expect it to. I'm thinking that the problem has to either be there or else there isn't sufficient vacuum pressure on the line from the #3 cylinder. This seems unlikely, though, because everything looked great when I synced the carbs (with a four dial guage tool).

Or a leak in that hose...?

 

UPDATE:
I swapped a known working petcock from my other XJ650 and replaced the vacuum line. The problem persists.

I've got an in-line fuel filter that makes the line go just slightly "up hill" before it drops down to the carb. I didn't think that this would be a problem so long as there is fuel in the tank above that level. Plus, fuel flows just fine when the valve is in the "prime" position.

 

I know it is probably not easy to check but is there a blockage in the vacuum line? Seems silly but it sounds like there is no vacuum there. A way to check is to run the motor with the vacuum line off the petcock and see if you are getting suction. If that line was leaking you would be seeing other problems with the running of the motor. Just something to check, even thou you have probably already done that.

 

Perhaps the plunger is a little longer than it needs to be and isn't opening far enough. Check and make sure the metal washer isn't slightly bent. Mine was a little too short and I had to slightly bend mine to make it shut off when the engine was off. I put the plunger into a small socket and lightly tapped it to make a very slight demple in the metal washer. I guess it could work in reverse.

 

If you haven't already, check the spring that seats onto the plunger. When I rebuilt my peacock the spring kept coming off the plunger as I reassembled the petcock.

 

Diaphragm, plunger, spring all seated and operating properly...on both petcocks I tried, (see my second post). The fact that this behavior is duplicated on a second petcock (known to be in working condition) means that it has to be either a vacuum issue or some problem with the fuel line that I don't understand.

Thanks anyway.

 

First check if your tank will drain fuel with the petcock off. you may have blockage in the tank. if that does not be the trouble check if you have fuel flow with the switch in the pri position. If that is ok check vacuum with bike running if you can. If that checks ok check drawing simple vacuum with a suction device of some kind. only if your fuel filter being quite a bit smaller than the fuel feed hose. fuel thru the filter should not be a factor. If you have any more issues just shoot me a PM and I will walk you thru this. It should be a very simple fix.

 

It is not uncommon to have complications arise when attempting to overhaul a Stock Petc0ck.

Despite how meticulously you follow procedures some aftermarket kits may be ill-fitting.

The Plunger may not travel.
The Plunger may not close.
The Wafer the Plunger is mounted too may be to large in diameter.

Compare the Parts of the Kit to the Original to see if they match-up.

 

As previously stated, fuel flows just fine with petcock in the "prime" position. Bike runs great, in fact. That also eliminates blockage in the in-line filter.

 

try hooking the vacuum line to the #1 vacuum port, or even sucking on the vacuum tube and see if that works. It seems that you have eliminated everything except the vacuum operation.

As a side, I had a vacuum port on an auto throttle body become plugged with some sort of residue which looked like clay. It was part of the pcv system and prevented the pcv system from working. You might want to probe your vacuum port gently with a small wire.

 

Be sure that the Hose from the Manifold Vacuum Port to the Vacuum Nipple on the Petc0ck IS ACTUALLY ::: Vacuum Hose!

Non-vacuum hose will collapse on itself when warmed up.