yet another vacuum petcock question.

I have been doing my research but still don't fully understand my petcock and its malfunction. I am the proud owner of 2 81 kz650csr's, my first has the classic petcock problem where it constantly flows fuel at any position, it has been rigged with the oh so ugly inline manual on/off switches from an auto parts store by the previous owner. The petcock on my other kz seems to work fine but it doesn't have the vac line at all and has no plug on the petcock or the carb, it seems to run ok but i don't understand how it is working without the vacuum line hooked up. My two questions are, can I run a manual petcock from an older kz? if so, what years, models? My other question is can I get away with not hooking up a vac line on my other kz? I'm working on the if it ain't broke don't fix it principle. I am not interested in rebuilding my petcock as it probably wouldn't fix it anyway. I know this has been discussed a million times but some people (me) are slow learners and i need some answers tailored my my specific issues.

thanks a bunch,
-Calvin

You should be able to run a manual petcock without any problems, but if you do you should cap the vacuum port on the #2 carb, and don’t forget to turn the petcock off when you shut off the engine.

The vacuum line allows the vacuum at the #2 carb to open a diaphragm in the petcock which then permits fuel to flow. If the petcock is in the “Prime” position, this vacuum is not needed, but the petcock will not automatically turn off the flow of fuel.

If a bike is running ok with the vacuum line disconnected, either the petcock is turned to the “Prime” position or the diaphragm is damaged (very likely) and letting fuel flow all the time regardless of the lack of vacuum. In that scenario, you should cap the vacuum port on the #2 carb so it doesn’t suck in raw air and possibly cause a lean condition in #2 cylinder, and cap the vacuum port of the petcock to prevent a possible leak. If you run like this the petcock will not automatically shut off the fuel when the engine is stopped. The risk of running it that way is the possibility that one or more of the carb float valves will not seal 100% in which case fuel will overflow from the carb bowl either through the overflow tube (if your carbs are equipped with them) and/or into the cylinder. In the latter case, the fuel will contaminate your motor oil.

It sounds like an ON/OFF valve was added to let you shut off the fuel, so the overflow problem shouldn’t be a problem as long as you remember to turn off the fuel when the engine isn’t running. But it would be a good idea to cap the vacuum port on the #2 carb, and I would definitely cap the vacuum port on the carb, otherwise if it decides to leak you could have a fire hazard. Ed

Take the petcock apart for inspection. If it appears to be in good condition, clean it w PineSol & hot hot water. Buff the surfaces w steel wool or brass wool, especially where the diaphragm o-ring contacts the petcock body. Then install rebuild parts from Kawasaki or get a K&L kit from Z1Enterprises.com. Mine was in good condition, w no pitting or scratches or anything & the kit worked.

The 650-CSR bolt hole spacing is 34mm. KZRider - Manual fuel valve option
If the Yamaha unit mentioned in the topic fits, I think they're about $20 at RonAyers.

There's a line of reasoning in your question that makes me want to add this:

With a couple of exceptions, all of our motorcycles use a gravity feed fuel system (fuel injected Vulcans do not). The petcock is there exclusively to turn the flow of gas on and off. It's not a fuel pump (which is a common misconception) so if there is no vacuum, no fuel should flow in the 'on' position. In the 'prime' position, fuel flows all the time whether there is vacuum or not. If you have a "Res" position, it probably runs all the time (like the 'Pri' position).

The Reserve tube on many bikes has been modified to be open to the tank, allowing stuff that the normal petcock intake mesh to flow into the carbs. This is why I always advocate a high-flow, low-pressure fuel filter in line from the petcock to the carbs.

If you can get a petcock that will physically fit and matches your tank configuration, you should be able to put any of the aftermarket ones you like on the tank. If you select a manual (versus vacuum) petcock, you will need to turn the gas on and off when you get on and off. That should be the only difference from one to the other.