Leaks gas while on kickstand

[System9]

When I set my bike on its side stand, it leaks gas from the air filter and possibly the overflow hoses. While on the center stand its not really noticeable. Now, I already have an ongoing list of maintenance procedures in the works, but I am trying to understand exactly what is causing this, and also I'm somewhat unclear on how the vacuum controlled fuel petcock works, or malfunctions for that matter. So, to isolate the problem, here is a question.
lets just say that my fuel petcock and carbs are not faulty.
If i put the petcock on the PRIME setting while the bike is turned off, will fuel eventually start overflowing from a healthy set of carbs? Or would the fuel only overflow from the carbs if there were a problem with the carbs?

[mrbill5491]

It is your petcock, being that its the vacuum type (right) then that is where the issue is. Never leave the petcock on prime, it will flow fuel to the bowls and the needle mostly likely can't stop the steady flow of fuel. If you can I would suggest replacing the vacuum set up with a strictly manual petcock where you can have a positive shut off for the fuel.

[mmain1]

...If i put the petcock on the PRIME setting while the bike is turned off, will fuel eventually start overflowing from a healthy set of carbs? Or would the fuel only overflow from the carbs if there were a problem with the carbs?

The general consensus here in the forums seems to be that the floats in the carb will not hold back fuel forever. I have seen people say that 15 minutes is about the threshold that the floats will hold back fuel from an open petcock (in Prime). I think the floats should hold back fuel indefinitely if they are set properly and there is nothing wrong with the needle/seat and the float height is correct, but that doesn't seem to be the case after listening to the veteran GS'rs here.

Another consideration though is how the petcock fails. I think more often than not what happens is the diaphragm in the petcock fails and fuel goes down the vacuum line (which should never have fuel in it), straight to the venturi and then down into the cylinders.

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[tkent02]

If i put the petcock on the PRIME setting while the bike is turned off, will fuel eventually start overflowing from a healthy set of carbs?

maybe, maybe not, sometimes, do you feel lucky?

Just replace the petcock, it's easy.

[AJ]

The general consensus here in the forums seems to be that the floats in the carb will not hold back fuel forever. I have seen people say that 15 minutes is about the threshold that the floats will hold back fuel from an open petcock (in Prime). I think the floats should hold back fuel indefinitely if they are set properly and there is nothing wrong with the needle/seat and the float height is correct, but that doesn't seem to be the case after listening to the veteran GS'rs here.

Another consideration though is how the petcock fails. I think more often than not what happens is the diaphragm in the petcock fails and fuel goes down the vacuum line (which should never have fuel in it), straight to the venturi and then down into the cylinders.

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I agree. Technically, the floats and fuel needle valve *should* stop the fuel, but that isn't their main purpose, which is why older bikes had a manual fuel shutoff and newer carbed bikes have a vacuum-operated petcock to make it "idiot proof."

Is the fuel leaking out of all the carbs or some of carbs? Hard to tell if you have a stock airbox installed, so pull the airbox back and have a look. The carbs on my bike leaked back through the pod filters on 3 of the carbs and I ended up replacing floats, seats, and needles when just adjusting the float heights wasn't enough.

The good news is that if fuel is leaking back out the airbox, then it's probably not flowing into the engine and diluting the crankcase oil. But this still not only a performance issue, but also a safety issue that should be fixed.

[System9]

I feel like I want to put a new petcock on, but the kind with a manual on/off setting. The fact that a faulty vacuum controlled petcock can allow gas to run through a vacuum line and into the engine is seems too risky to even be messing with.
Another question, if my petcock is bad, would it cause my carbs to overflow even when the bike is running? Because the gas leak happens when the bike is running too.

[mrbill5491]

Oh the leak happens when it is running too. You should have mentioned that in your first post. Sound like you have float level issues as well. Needle and seat could be bad, floats not adjusted right, floats are damaged and are fuel soaked, another can of worms.

[tkent02]

I feel like I want to put a new petcock on, but the kind with a manual on/off setting. The fact that a faulty vacuum controlled petcock can allow gas to run through a vacuum line and into the engine is seems too risky to even be messing with.
Another question, if my petcock is bad, would it cause my carbs to overflow even when the bike is running? Because the gas leak happens when the bike is running too.

The vacuum units work fine for decades, it's when they sit for years that they fail.
I have some bikes with manual petcocks, some with automatic ones, some with none.
Guess how often I forget to shut off the gas on the ones that need it?

If it overflows while running you have float or needle valve problems, too.

Sounds like everything in there needs work, that what happens when they sit neglected for decades.

[mmain1]

...Sounds like everything in there needs work, that what happens when they sit neglected for decades.

+1.

You should plan on being without it for a little while and take some time to rebuild the carbs following the guide (I think it's this one)

And get a new petcock on order.

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[System9]

Yeah, its an ongoing project, was in rough shape when i got it. I already have the new o-rings / seals for the carbs on order, and have downloaded and printed out the complete carb rebuild guide, which includes dipping the carb bodies. I was just trying to figure out ahead of time if i had an issue with the petcock, the carbs, or both.
The circumstance that still has me wondering though, is why does it leak while on the kickstand, but not while its on the center stand?
And thanks everyone for all the good info.

[DanTheMan]

The carbs most prone to leakage is #2 and #3 because it normally sits on the sidestand, and that's where the fuel enters the carbs. Whenever my carbs want to leak they do it while on the side stand.

[sedelen]

This has been and will continue to be an ongoing issue on the forum (right Koolaid Kid?). Most owners manuals have a caution about leaving the petcock left on "Prime" simply stating that to do so may allow your carbs to overflow. To me that doesn't necessarily imply there's problems with the carbs.
Both my bikes carbs will overflow if I leave the petcock on "Prime" as they have done before when I got careless and forgot to switch it back, and both were on the sidestand. I have not had them overflow and have not left the petcock on "Prime" since.

[System9]

I've never left mine on Prime either, thats why I was wondering if my petcock was bad.
But tonight I tore into it and did some testing. I've decided the petcock is bad, and that sucks, but its not as weird as the carb leak.
First thing I did was opened both carbs, and checked all the parts of the float mechanisms. The float needles were pretty springy, and I have cleaned these carbs a couple times, and they looked shiny clean inside. I swapped the float needles just to see if the problem would switch to the other carb, (it didnt)
I put it back together and hooked up a gas tank from a Honda CM400
I covered the vacuum nipple on the carb.
With the bike on its center stand, i opened the petcock on the Honda tank and watched the fuel flow through the fuel filter and into the carbs.
After a minute the carbs were full, and not one drop leaked from the air filters or over flow hoses.
I started it up and it idled great, I let it warm up, took it off choke, and everything seemed perfect.
Then i was feeling optimistic so i put it back on the side stand and almst immediately gas came shooting out of the right carb overflow hose. This is the carb opposite the kickstand, so at this point it it the highest elevated carb.
The left carb isn leaking at all.

[mrbill5491]

Pull your floats out and weigh them. If the one that is leaking is heavier means its is not seating the needle and flowing fuel. Your float has a flaw in it and is fuel soaked thus making too heavy for it to work right.

[System9]

Great Idea, I actually shook them last night to see if I could hear gas in them, but I'll use my electronic scale.

Also.....and this is weird and somewhat frightening.

Last night, in preparation for my carb dipping, I went to remove the needle jet, and It wouldnt budge. The carb guide says to "gently remove with pliers" well, gently was having absolutely no effect. I also didn't want to damage the little rubber o-ring on the end of the needle jet with the pliers.
Thats when it hit me. The picture of the needle jet in the guide doesnt have any o-ring, at least not visible until it is removed. Also, there are no tiny holes visible on my needle jet. And then it dawned on me. maybe...

The previous owner put the needle jets in UPSIDE DOWN?!!!

I will take a photo tonight to make sure. I also don't see a way to remove them without damaging them.