fuel leaking out overflows with gas turned off?

[klrx7]

I have an xs 750 tank with original vacuum petcocks. I run them in the pri position when running and in the on position to turn them off. After a ride I will park the bike, turn the fuel off and then a few hours later fuel is leaking out the overflows and continues to bubble out for the next day or so. I have rebuilt the petcocks and when the tank is off the bike and full of fuel it does not leak at all with the fuel shut off (on position). Vacuum lines are open to air btw. Is this common? is there a fix other than replacing the petcocks? Tired of walking into my garage full of gas fumes.

 

[Brassneck]

Hmmm...interesting way of using the vacuum petcocks, but I suppose if you don't want to use the vacuum portion, running them the way you are wouldn't be a big deal...but the issue is you do have a problem...or, more likely, with them being run on Prime...and then turning it to "on" (which essentially is off, the way you are using them), I'm wondering if you are filling the carbs so full that they can't burn off fast enough and the bowls get overfilled? That's just a guess...

Another issue I would foresee with running them on prime the whole time is you wouldn't be able to prevent a low fuel/reserve situation before it's too late...unless you monitor the mileage really closely.

Is there a reason you don't want to use the vacuum lines the way they were intended? (Just curious)

 

[klrx7]

brassneck, it is on a 76 650. Cone filters - hence no vacuum lines. It is almost as if there is pressure in the lines between the petcocks and carbs that builds after the bike sets for a while and over powers the float needles. what doesn't make sense is last night I drained the left side bowl and this morning a puddle of gas again and when I barely shake the bike from side to side gas bubbles out both overflows. Float needles should be preventing any excess fuel. I guess I will tear into them and try to get them to seat better. And try to find some different petcocks too.

 

[Brassneck]

Gotcha. Ok, since "on" really isn't a true "off", maybe the vacuum hose has something to do with it. I wonder if there's some back pressure that is building up? Maybe try plugging it off?

Or, I wonder if the tank itself is building up pressure and forcing fuel out...not right away but over time? (Do you get a "HISS" sound when opening the tank by chance?)

If it were me, I'd pull the fuel lines and see if there's a really slow trickle from the petcocks...let them sit over night (with a cup or alternate line attached so it doesn't go into the carbs). If so, I'd get non-vacuum petcocks.

 

[klrx7]

just did that what I noticed is no fuel leaks when I pull the fuel lines off but if I lay my finger against the nipple fuel runs right out over my finger. almost as if the vacuum pump is an air vacuum stop and doesn't actually seal the fuel off. I thought it sealed it off when open to atmosphere and opened under vac. maybe it's a combination of petcocks not actually turning off with a fuel line connected and faulty float needles.

 

[Brassneck]

Could be all those. My guess for truly fixin it is to get non vacuum petcocks (I'm in the same boat...750 tank with petcocks that are holding for now, but likely will switch when they stop working) I think the standard '78 xs650 petcocks fit the xs750 tank, IIRC..of course the true cure is to just keep riding it.

 

[grizld1]

From the Dictionary of Motorcycle Terminology, Crasher Burns ed. (Coventry, 1985):
Vacuum Petcock: A device engineered in Hell to Satan's specifications, with a primary function of luring owners of motorcycles to acts of rage and despair conducing to the perdition of their eternal souls, and a secondary function of topping crankcases up with fuel.

I used the manual petcocks from Mike's XS on a Special tank years ago, and they delivered years of good service, but I don't know about the current batch; things there don't seem to change for the better. Lowbrow Customs sells some nice 1/4" NPT screw-in petcocks for around $16 each, and Michael Morse sells 1/4" NPT Motion Pro units. Prices on adapter plates from Pingle are insane, but it's easy to make your own; I use 5/16" bar stock when I can find it, 3/8" when I can't. Good luck, guys!

 

[XSLeo]

Using cone filters won't prevent using the vacuum line for the petcock. The vacuum port for the petcock is between the carb and engine. The cone filter is on the other end of the carb.
Leo

 

[vsop-dk]

From the Dictionary of Motorcycle Terminology, Crasher Burns ed. (Coventry, 1985):
Vacuum Petcock: A device engineered in Hell to Satan's specifications, with a primary function of luring owners of motorcycles to acts of rage and despair conducing to the perdition of their eternal souls, and a secondary function of topping crankcases up with fuel.

hehe , good laugh this one. Thanks.

I would make an Exchange of petcocks, to get you out driving without dizzyness og the fumes.

 

[gggGary]

You can epoxy the vacuum part of the petcock shut. XJWMX said he did it.
check your cap for proper venting? The rubber gaskets must be replaced with new the old ones are hard and will not reliably stop fuel flow. The face of the selector needs to be perfectly flat and smooth, sand by using a rotary motion on fine (600) grit paper on a flat surface, then polish.

 

[dps650rider]

The problem is likely that the vacuum passage in the petcocks is partially plugged and the vacuum is not releasing. I had that problem and figured it out a few years ago and posted information on it in one of the other forums. Once I cleaned these out never had another problem.

I'll take a look and see if I can find the post.

 

[dps650rider]

Here is a link

http://xs650temp.proboards.com/thread/15112/vacuum-petcock-problem-solved

 

[dps650rider]

Here are some pictures: