Leaky petcock -- how does vacuum leave petcock?

Hi everyone, first post here, hope it's alright.

The petcock on my 84 KZ750L (Canadian here) is leaking; full flow when vacuum not applied.

Thing is though, I took it apart, and it seems the rubber is all good. The only reason it's sticking open is because when vacuum is applied, and then removed, the vacuum 'chamber', stays, well, vacuumed. The vacuum pipe connection allows air out, but not in, which means it's having trouble equalizing, no matter how strong the spring (I've tried making the spring a little longer).

Now, I guess my real question is, how is the petcock vacuum chamber supposed to work? Is the connection supposed to only allow air out? I've got it soaking in carb cleaner right now, just in case it's some varnish or something.

There is a small vent hole on the non-vacuum side of the diaphragm that may be blocked. Could also be the small o-ring sticking. Either way, a careful cleaning may fix the problem. If not, new rubber parts may be in order.

Problem solved! Can't say I did anything but be gentle and wipe out said vent with a wire and a rag. There really wasn't anything blocking, but I suspect the rubber was getting bunched up there, and probably something to do with getting it from the inside of the membrane to the outside/vacuum side.

Bottom line, being aware of that vent helped me fix it. Thank you!!

So, just an update on this. The problem reoccurred, and then when I went to home depot and came back with some hose that fits on the petcock to drain the tank again (prepping for a new diaphram now), I came back to the petcock shut off just fine...

I believe the problem to be a lack of seal in the diaphram, which allowed fuel to the vacuum chamber, as well as between the diaphrams (washer section, where the vent is). On top of, I'm sure, being bad for my intake having bits of fuel sucked in through the vacuum line, the fuel was probably flooding the vent, causing a non-functioning petcock until it drained away.

New diaphram on order, and all should be well after that.

Yea, I've fought the logic how it works, or why it doesn't and found if it doesn't replace the diaphragm and it works every time. Some guys have had not working petcocks even after diaphragm replacement though, so good luck.Use the Kawasaki factory diaphram though if still available, as the aftermarkets won't work.

I've found that, being in Canada, shipping makes the dealership 100% the best and cheapest option for any *new* parts. The diaphragm is used all the way up to the Concours 1000, which ran until what, 2004? So petcock parts are super easy to get.

Well, bad news, a brand new diaphragm set (which included the spacer; came assembled) didn't help.

Logically, I still can't see how that vent could possibly help unless the vacuum-side diaphragm was leaky.

The assembly as far as I can tell is:

Vaccuum chamber, diaphragm, vented chamber between diaphragms (that's the spacer), diaphragm, valved fuel flow chamber.

Unless there's a leak between the vacuum chamber and the vented chamber, it's impossible to equalize.

I don't understand why there's a valve that only allows air out of the vacuum chamber, wouldn't the vacuum pulses alone be enough to keep the fuel flowing? It seems like a solution would involve drilling a tiny hole in the back plate of the petcock...

The vacuum goes away when the engine is off.
Steve

haha, yes, of course, the supplied vacuum stops when the engine stops. The problem is the petcock's vacuum chamber (here www.cmsnl.com/kawasaki-1983-kz750-l3_mod...461005/#.VUq5ZyvF948 ) 'stores' the vacuum with a valve (on closer inspection, it looks like a tiny reed valve).

There's only one hole in that part, that I can see, and it only lets air *out* of the chamber. I'm sure I could just order the part to see if that's the issue, but I don't really want to just throw parts at the problem.

Is there a particular orientation for the diaphragm assembly maybe? Logically it just makes sense to me that there should be a tiny equalizing hole in the chamber I posted above.

Any hole in the chamber would be a vacuum leak. It has to be sealed. So the way it works is more or less like Steve said, shut the engine off, there is no longer vacuum to suck the diaphram, and the spring pressure pushes the diaphram to closed position. No way for vacuum to remain after the engine is off because the vacuum line down to the carb is wide open in the carb releasing all vacuum to atmospheric pressure in the case the engine is not pulling vacuum.

Couldn't have said it better.
Steve

The vacuum port for the petcock has a valve that is not allowing the vacuum chamber to equalize with the vacuum-free carbs. My understanding is that vacuum lines have pulses of positive pressure, which would allow the diaphragm to close while running. The valve ensures that only negative pressure is used.

Have you ever noticed this valve, or is there debris acting like a valve?