Vacuum petcock still passing fuel when engine is off

Port

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ive got an 1981 xs650 sh with the stock vacuum petckock. I rebuilt it a few months back but just poot fuel in it for the first time. It doesn’t leak but it is operating like a gravity fed petcock in all 3 positions. Anyone know if its possible to get this result if reassembled incorrectly? And if so what I might have done. I remember thinking there was only one way that it went back together but I might be wrong about that. Figured I’d ask before opening it back up.
 
ive got an 1981 xs650 sh with the stock vacuum petckock. I rebuilt it a few months back but just poot fuel in it for the first time. It doesn’t leak but it is operating like a gravity fed petcock in all 3 positions. Anyone know if its possible to get this result if reassembled incorrectly? And if so what I might have done. I remember thinking there was only one way that it went back together but I might be wrong about that. Figured I’d ask before opening it back up.

Hi Port,
seems like the petcock itself is OK but it's vacuum on/off part ain't working.
The search button will find you the way to fix that.
You can also file the nubbin off the front plate and cut the petcock lever short so it'll point straight up to give the petcock a true OFF position.
 
Perhaps the spring that closes the diaphragm is broken or missing, or there is some dirt preventing the diaphragm valve from closing.

If you have access to a lathe it is quite easy to make a new lever. Cut the fuel channels in the lever to make a mirror image of the old one. Then seal the diaphragm valve, remove the plastic square and seal the vacuum hose. This will give Reserve = Reserve, ON = ON and Prime = OFF.
 
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ok yeah i definitely lost the spring during the rebuild and didn't notice. Anyone know where i can get a replacement? I have the style of petcock with a non-removable outlet pipe and a drain screw. i saw in this post ( https://www.xs650.com/threads/rebuilding-yamaha-vacuum-operated-fuel-valves-petcocks.20662/ ) that there are 2 different springs. but it's not clear which is for my style of petcock. in addition to that i cant find them anywhere on line and the only part number i can really find is for a much much larger spring.
 
the spring in the background of this pic is the one you need.
B4-N-Aftr_resurf.jpg

Couple fine points, you mention rebuild, did you buy an aftermarket kit?
I have had miserable results with aftermarket parts; I only use genuine Yamaha valves (four hole rubber discs) the aftermarket tend to be thin and leak.
I found the exact same issue you have, and after lots of hair pulling, found the diaphragm plunger on the aftermarket kit was just a little shorter than the stock plunger. My most recent forays into petcocks have involved oh so carefully putting the large disc on the end of a deep dish socket and slightly "dishing it" to in effect make the shaft a touch longer.
DSCN3795.JPG DSCN3789.JPG DSCN3792.JPG
I've said several times that a smooth or even polished surface on the selector and the seat that the vacuum plunger o-ring rides against are critical to a valve that doesn't leak.
That "non removable" exit does usually need to be removed and have fresh epoxy applied, make sure all the old epoxy is scraped out of the ring you can see on the exit elbow.
bs34 petcock (7).JPG bs34 petcock (6).JPG .
 
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So I went looking for a spring in the bins and came up with some gotchas; two of the petcocks used a shorter fatter spring one had a longer but skinnier spring. The short fat springs are from (not XS650) petcocks, prolly XS750 or the like. some pics showing the slightly different vacuum covers and springs. Not 100% that skinnier longer spring is original it was in with a disassembled 80-84 XS650 petcock, Also noted from a thread elsewhere; check the check valve located in the vacuum tap fitting. it should allow the engine vacuum to pull the plunger open but return and seat slowly once there is no vacuum, over say a few seconds.
DSCN3791.JPG
 
I included a couple of pictures of my petcock just to be clear. It looks like the one on the left that you posted is correct. Im not sure what the spring is supposed to look like, ive never pulled another apart. I pulled the outlet out and cleaned off the old epoxy. Any suggestion as to what epoxy to use to replace it? I have some red-kote that might do the trick. I intend to polish up the critical surfaces before reassembling it.
IMG_0722.jpg IMG_0723.jpg IMG_0725.jpg IMG_0726.jpg IMG_0724.jpg
 

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