On, Res, Pri......

[MagicRat]

I am used to bikes with fuel petcocks with ON, RES, OFF as the 3 choices.

On my newly acquired '81 Yamaha XJ 650, it has ON, RES and PRI as the 3 choices.
What does the PRI setting do, as it does not seem to shut the fuel off?
(Of course, this bike has a simple gravity fuel feed to the carbs. )

I also noticed there seems to be a vacuum line going from the petcock to the #2 cylinder intake manifold and the petcock seems to have a vacuum diaphragm.
Is this a vacuum operated fuel shut off?

It would be nice to be able to shut the fuel flow off (slightly leaky carb), which as present does not seem to happen, regardless of the petcock setting.

[coppersv1000s]

Originally Posted by MagicRat

I am used to bikes with fuel petcocks with ON, RES, OFF as the 3 choices.

On my newly acquired '81 Yamaha XJ 650, it has ON, RES and PRI as the 3 choices.
What does the PRI setting do, as it does not seem to shut the fuel off?
(

maybe prime? primary? i dunno. sorry

[richtazz]

Yes, it is a vacuum operated fuel shut off. PRI is the primary position, and when working correctly, doesn't need to be touched. On is there so you can override the vacuum part and still ride if the diaphram fails. RES is self explainatory. The default function of the petcock in PRI is off, which is supposed to stop fuel flow except when engine vacuum is present. The spring inside the petcock must be bad, and not pushing the shut-off pintle up causing your leak. It's not a very good design IMO, something very un-Japanese.

[jeffcoslacker]

Originally Posted by richtazz

Yes, it is a vacuum operated fuel shut off. PRI is the primary position, and when working correctly, doesn't need to be touched. On is there so you can override the vacuum part and still ride if the diaphram fails. RES is self explainatory. The default function of the petcock in PRI is off, which is supposed to stop fuel flow except when engine vacuum is present. The spring inside the petcock must be bad, and not pushing the shut-off pintle up causing your leak. It's not a very good design IMO, something very un-Japanese.

First time I'm gonna disagree with you, Rich, but I may be wrong....but I don't think so...you've just got it reversed.

this is the same as the '83 XJ650 I currently ride. It is a vacuum petcock, but the ON position supplies fuel when engine vacuum is present, PRI I assumed to be PRIME, because it defeats the vacuum cut-off...sometimes if I haven't ridden the bike for a few weeks the carbs will dry out and it won't start...I have to put it on the PRI position to refill them, and it starts right up...

ON is the regular riding position, and will prevent fuel leaks if the carbs were to get stuck while sitting, so unlike older models without the vacuum petcock, you won't find half a tank worth of gas all over the ground running out of the overflow tubes....

[jeffcoslacker]

PS I do agree the vacuum mechanism is broke if it leaks with the motor off in RUN or RES...

[coppersv1000s]

Originally Posted by jeffcoslacker

First time I'm gonna disagree with you, Rich, but I may be wrong....but I don't think so...you've just got it reversed.

this is the same as the '83 XJ650 I currently ride. It is a vacuum petcock, but the ON position supplies fuel when engine vacuum is present, PRI I assumed to be PRIME, because it defeats the vacuum cut-off...sometimes if I haven't ridden the bike for a few weeks the carbs will dry out and it won't start...I have to put it on the PRI position to refill them, and it starts right up...

ON is the regular riding position, and will prevent fuel leaks if the carbs were to get stuck while sitting, so unlike older models without the vacuum petcock, you won't find half a tank worth of gas all over the ground running out of the overflow tubes....

well hell either way i was close!

[MagicRat]

Originally Posted by jeffcoslacker

this is the same as the '83 XJ650 I currently ride. It is a vacuum petcock, but the ON position supplies fuel when engine vacuum is present, PRI I assumed to be PRIME, because it defeats the vacuum cut-off...sometimes if I haven't ridden the bike for a few weeks the carbs will dry out and it won't start...I have to put it on the PRI position to refill them, and it starts right up...

ON is the regular riding position, and will prevent fuel leaks if the carbs were to get stuck while sitting, so unlike older models without the vacuum petcock, you won't find half a tank worth of gas all over the ground running out of the overflow tubes....

Thank you everyone for your input. You have confirmed my suspicions about the function.
As it seems to pee fuel all the time, I was not sure if the ON or PRI position was correct, and I wanted to be sure before disassembling the vacuum mechanism.

Hmmmmm......... depending on how the petcock is made internally, positioning the lever midway between 2 of the settings might just shut the fuel flow off altogether....................

[richtazz]

I have no problem with being corrected Jeff. I believe you may be correct, and I may have my info backward. I was going off memory, and the older I get, the more jumbled that memory gets.

[72chevelleOhio]

Originally Posted by MagicRat

I wanted to be sure before disassembling the vacuum mechanism............

NNNNNNOOOOOOO!
I fought with one for a whole season. I could NOT get it sealed back up. I seen an entire valve for the XS a while back that was $107. I wound up using hose pliers and crimping all the lines off whenever I wasn't riding.....

[jeffcoslacker]

Originally Posted by 72chevelleOhio

NNNNNNOOOOOOO!
I fought with one for a whole season. I could NOT get it sealed back up. I seen an entire valve for the XS a while back that was $107. I wound up using hose pliers and crimping all the lines off whenever I wasn't riding.....

Yeah I think if mine went bad and was doing that, I'd just put an inline petcock in the fuel line and turn it off...or adapt a standard ON/OFF/RES petcock to work...

But I'm lazy too...and cheap....

[jeffcoslacker]

...of course it would be best to fix the carb that's leaking...if it leaks when shut off, it also runs rich when running...that float/needle isn't controlling fuel level...

If it idles rough or surges when cruising, that's why...

[MagicRat]

Originally Posted by jeffcoslacker

...of course it would be best to fix the carb that's leaking..

After much precise rebuilding and tuning ( and much whacking with a BFH) the carbs have stopped leaking and the engine runs............. well enough to hear the loose, flapping timing chain - the tensioner is worn, I think.

But the vacuum shut off is still screwed. I have already considered a tiny in-line shut off valve. Thanks for the input everyone.

[jeffcoslacker]

Does yours have the manual or auto tensioner?

The auto you sometimes have to turn the motor clockwise by hand a few rotations, then suddenly yank it back the other way, this makes the chain wad up and go floppy on the tensioner side and it'll draw up and take the slack out (if it's not stuck)...the manual ones is similar, but you have to take the cap off and use a screw/locknut type adjuster...it can't be too tight, or it will wear the chain quick...basically it should control flutter, but not be in full contact.

On the '83, the right side crank cover plate just has a big casting plug, but the left has the timing components and a nice big nut to rotate the motor with...if your ears are good you'll hear a click out the tensioner (auto) when it adjusts up...

[jeffcoslacker]

XJ chains rattle a lot at idle regardless...the primary chain makes noise too...

Been here yet? Good stuff...

http://www.xjbikes.com/