Petcock on off question..

[Jpwhit1]

I've tried so hard not to ask this dumb question.... What is "off" on the petcock. I was having problems with gas pouring out of filter until I did a thorough cleaning. While I was having that problem it didn't matter what I was turning petcock to, it would not stop supply from tank. Maybe I need to rebuild them.? Main question though, what is off! Thanks

 

[weekendrider]

There is no off position as supplied OEM.
If you have the 2 petcock 79 special tank try swapping the selectors.
http://en.650wiki.org/index.php/5.02.1_Vac_Petcock_Conversion

 

[5twins]

You have the later vacuum type petcock and as WER said, there is no "Off" position. But, if operating properly, the "Off" position isn't needed. Your petcock shouldn't flow any fuel in the "On" and "Res" positions unless the motor is running and providing vacuum to that little fitting on the backside of the unit. A small diameter hose should run from there to a nipple on one of your intake manifolds. The "Pri" or "prime" position will flow all the time, by-passing the vacuum powered portion of the petcock.

To test whether or not your petcocks are operating properly is easy. Simply disconnect the fuel line from the unit. No gas should come out with the petcock in the "On" or "Res" position and the motor off. Start the motor and gas should start coming out. Gas will come out in the "Pri" position any time, motor running or not.

 

[XSLeo]

In your signature you list two bikes, a 71 XS1B, it uses a non vacuum petcock. it has three positions On/OFF/RES.
In the OFF position no fuel should flow.
On your 79 it has either one or two, not sure, I don't have one, vacuum petcocks, As described by WE and 5twins there is no OFF. If yours don't work as described you can get rebuild kits.
I've tried rebuilding vacuum petcocks with a 50/50 chance of proper function.
Mike's sells a replacement regular petcock that fits on a vacuum petcock tank. The mount bolts are farther apart on the vacuum petcock. You can find them on Ebay as well as other sellers.
Pays your money takes your chance.
Leo

 

[Jpwhit1]

Thanks guys!