How would a petcock allow half a tank of gas to run out while bike is shut off ? - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 6 (permalink) Old 10-07-2009, 12:10 PM Thread Starter
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Question How would a petcock allow half a tank of gas to run out while bike is shut off ?

I just read a post by joe, from Tampa, about his bikes carbs catching fire.
https://www.vn750.com/forum/showthread.php?t=12915

He relates an earlier problem where the carb filled the rear cylinder with gas, and then spilled half a tank of fuel on the ground. I thought he must have an early model with a "prime" position on the petcock, which would allow gas to flow without vacumn applied to it. This does not seem to be the answer, as earlier posts indicate he has a 1994 model, with the "off-run-reserve" petcock which requires vacumn to the middle nipple to allow fuel to flow to the carbs.

I do not understand what mechanism would allow more fuel than what was in the carb bowls, to leak past a stuck float and to run into the cylinder or on the ground, while the engine is shut off.

How does one explain this happening?

Gordon

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post #2 of 6 (permalink) Old 10-07-2009, 12:35 PM
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a faulty diaphram could fail in two ways ... one leaks past the stop in the diapsham and then vac wouldnt need to be applied and 2 allowing gas to slip into the vac port and down into the carb this way.... wouldnt take long for a stuck float to leak enough gas to start a fire so if the petcock was working and the bike got started it could spill enough on the ride to wherever he was goin ...

also if the tank was just overfilled and the over fill hose wasnt attached its likely for fuel to leak on the rear exhast manifold...



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post #3 of 6 (permalink) Old 10-07-2009, 12:39 PM
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IMO, it has to be a faulty petcock with a damaged diaphragm that allows that to happen. I'm not a big fan of vacuum operated petcocks to begin with, although it really is nice not to have to turn your petcock off every time you shut off your bike, just in case. You never know when a float valve might malfunction and if the petcock is leaking gas with the engine turned off, then you can have gas going everywhere. I put an on/off petcock on my KZ's just cause they were older and I didn't trust their vacuum operation, plus I had some float needle problems once.... As much as all of us don't like to rotate the lever on our petcocks, it is the only way to know the gas is not going to leak past it, unfortunately.

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post #4 of 6 (permalink) Old 10-07-2009, 12:47 PM Thread Starter
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Thanks for the explanation. I haven`t ever worked on a petcock, and do not really have a clear picture of how everything moves, and is put together.

Gordon

1991 VN 750 -"Cosmic Lady" or "Bad Girl"?
Purchased May 16, 2008
Approx.19,300km (12,000 miles)

H-D windshield
Relocated R/R
MF-AGM battery
Fiamm Freeway Blaster horns
F&S luggage rack and engine guard
Kury Offset Hiway pegs
July 13, 2016, Riding on the DARKSIDE now, Classic Radial 165/80-15


TOP TEN THINGS A NEW RIDER/OWNER SHOULD DO. Click on link.
https://www.vn750.com/forum/showthread.php?t=9127
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post #5 of 6 (permalink) Old 10-07-2009, 07:03 PM
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This is why kawasaki change the petcock so when you are not riding you can turn the fuel off. That way if you have a faulty diaphram or float valve the fuel loss will be minimal. In his case if he would had made a couple of cheap repairs he would still have his bike, I have yet to see any fuel leaks correct themselves.


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post #6 of 6 (permalink) Old 10-07-2009, 08:51 PM
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And the really cool thing is, on a bike, a fire generally starts right under the gas tank! Woo hoo! Yep, as soon as I saw a fuel dumping issue on my KZ's it was "Katy bar the door" until I had it resolved. It will get your attention, well it should!

Fergy
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