Can I leave the petcock open???

[clyde the ape]

Well basically title says it all.  I was told this is why my carbs are leaking. Also if the bike is leaning they could leak.  So I have to close the petcock all the time & I can never use the side stand? I have to jack it up on the center stand everytime ?!?!?  Is this for real??  I am starting to smell something..............

[rrgunslinger]

I turn mine off everytime I stop.

[clyde the ape]

I know it is good practice to turn it off.  But is it that crutial??

[Piute]

                                                               

[shinyribs]

I turn mine off when I come home and park for the night. A few times I have forgot and there was no problems of leakage even after sitting for a few days at a time. I know some guys never cut the gas off. it shouldn't leak on the side stand. If it does there are problems elsewhere.

[rrgunslinger]

Guess not if you are sure it will not leak.  I just don't trust them (carbs).  I have owned two of these now and they all leaked at one point or another.  All it takes is a little trash in the float needle.

On Wed, Jun 6, 2012 at 6:27 PM, clyde the ape [via Honda CB750'S] <[hidden email]> wrote:

I know it is good practice to turn it off.  But is it that crutial??

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[kschneider90]

i would turn it off i had a problem with my float and carb gaskets deteriorating.

[sgtslag]

It depends on the year...  The 1981 Custom's had a vacuum-controlled, petcock:  auto-magic, cut off the gas every time the engine stopped, no muss, no fuss.  The earlier models had a manual valve, which is supposed to be turned "Off", every time you shut the bike off, as the float bowls can fill, and overflow (gravity feed from the tank), especially if any of the four needle valves are less than perfect.

As far as the side-stand issue goes, that suggests the bowls may have a leaky gasket?  I guess I would start by turning the petcock off, then use the side-stand.  If it leaks, you have an issue.  Start by tracing the source of the leak.  Be sure to check your inline fuel filter (OEM was on top of the petcock, within the tank), if you have one, as well as the petcock, itself.  Then check the fuel lines for leaks, and/or cracks.  After that, you are at the fuel path within the carburetor assemblies, and the float bowls.

My '79 gets left with the valve/petcock in the "On" position, quite often, as I forget.  I have not had a problem, yet.  I try to always turn it off, though, to be safe.  I do not want gasoline running down into the transmission, or the ground, puddling.  Older bikes tend to have more 'manual' controls, whereas modern bikes are more automated.  I prefer somewhere in the middle, but I love my vacuum-controlled fuel valve on my Voyager.
  Cheers!

[clyde the ape]

I will have to test the carbs again see if they leak with the petcock closed.  
Hey sgt any idea if I can put one of these "magic" petcocks in a 79 ltd??  Be nice if it was one less thing to worry about.

[sgtslag]

No idea.  It would require hooking up a vacuum hose to the carbs -- would need to add a splitter on an existing vacuum hose? It should work.  The real question would be regarding the Custom petcock's ability to screw into the gas tank's belly:  are the threads the same?  Most likely, they are.  If someone knows for certain about compatibility, please sound off, as I am working off theory.

For a better chance of getting a definitive answer, you may want to visit the CB750 Custom forum (registration required; not as friendly, by far, but the folks there are exceptionally knowledgeable; be sure to use the search function before you post your question!  They can be quite testy if you ask an old question that has been discussed, ad nauseum, in the past.  Best of luck.  Cheers!

[seestheday]

Didn't the 79 come with an auto-petcock already?  If it's not there then a PO could have just removed it.  I thought that they started in 78.
Lots do, and the cb750 custom forum that Sarge refers to below often call it the Spawn of Satan (SOS) because it can cause other issues.

I took mine off and cold starts improved massively.  It went from almost draining the battery before it would start to starting almost immediately.  I also feel like the bike is more responsive, but that could be due to me cleaning the carbs at the same time.

I still have mine, and have thought about putting it back on.

Right now, I just turn off the gas when I'm not riding, but I also don't have any issues with my carbs leaking and I keep them nice and clean.

[sgtslag]

No idea when the vacuum-activated petcock came out, other than my Dad's '81 Custom had it -- he replaced it with a manual cut-off valve.  My '79 had a manual cut-off when I purchased it, but my bike had at least two PO's, one of whom removed the fuel filter screen element from the petcock (had no fuel filter at all).  Like all doo-dad's, the automatic cut-off's are great when they work, and SOS when they don't.  Cheers!

[TOOLS1]

I never turn mine off. If they are going to leak, I want to know about it. If they leak while running, you run the risk of a fire. Also if they leak, they are broken, and need repaired. My thinking on leaving the the petcock open is that as long as the needle is seated it will not be able to get any crap, or build up on the seat.
TOOLS

[rrgunslinger]

That's fine leaving it on but the day you get up in the morning and there is a tank of gas in you garage on the floor you may change your mind.  :)  Just saying,,,

On Thu, Jun 7, 2012 at 1:22 PM, TOOLS1 [via Honda CB750'S] <[hidden email]> wrote:

I never turn mine off. If they are going to leak, I want to know about it. If they leak while running, you run the risk of a fire. Also if they leak, they are broken, and need repaired. My thinking on leaving the the petcock open is that as long as the needle is seated it will not be able to get any crap, or build up on the seat.
TOOLS

Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
1976 CB 750-A X 2
1977 CB 750-A X 2
1981 CB 750
1995 KZ 1000 P-14
1981 KZ 750 LTD X 2
1973 CB 350
1979 CM 185 Twinstar
1987 Honda TLR 200
1982 Honda XL 80
2005 Yamaha PW 50
South of Edan (Kansas City MO)

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[seestheday]

Good point, I just don't want to get stranded somewhere or stink up my garage with a huge amount of gas fumes because my entire gas tank leaked out.

I would also be a little embarrassed if I leaked a whole gas tank out in my work parking lot.

[Thorby]

A small puddle of gas will start forming under my bike after having been on the kickstand a couple of hours, when I forget the petcock on.  I'll follow your advice, Tools, suck it up, and take my bike to get that long needed carb maintenance.

TOOLS1 wrote

I never turn mine off. If they are going to leak, I want to know about it. If they leak while running, you run the risk of a fire. Also if they leak, they are broken, and need repaired. My thinking on leaving the the petcock open is that as long as the needle is seated it will not be able to get any crap, or build up on the seat.
TOOLS

[Lucky 1]

No you do not have to turn it off.
Matter of fact if the bike is sitting for a while (week), the carbs will stay full of gas.
Evaporation will cause the float to keep filling the carb.
Some times when the bike is on the kick stand the angle of the bike can cause a float to
stick but it is rare.


Just be careful when parking a motorcycle in a garage that has
a gas fired hot water heater close by.

IF you leave the gas on and a carb has leaking fuel you better put some
new float needles in the carb before you have a fire.

If you have to let a bike sit for 60 days or more it is good to drain the gas out of the tank and float bowls.
The new ethanol gas can break down after 60 days and all of the moisture will go into the bottom of the fuel tank and to the bottom of the float bowls and cause lots of problems.

[rrgunslinger]

I do agree with Tools that if they are leaking they need looking at.  I'm just not willing to take that chance over a long period when the bike just sits.  Even back in the 70's I made it a habbit to just turn off the gas on all my bikes. 

On Thu, Jun 7, 2012 at 2:48 PM, Thorby [via Honda CB750'S] <[hidden email]> wrote:

A small puddle of gas will start forming under my bike after having been on the kickstand a couple of hours, when I forget the petcock on.  I'll follow your advice, Tools, suck it up, and take my bike to get that long needed carb maintenance.

TOOLS1 wrote

I never turn mine off. If they are going to leak, I want to know about it. If they leak while running, you run the risk of a fire. Also if they leak, they are broken, and need repaired. My thinking on leaving the the petcock open is that as long as the needle is seated it will not be able to get any crap, or build up on the seat.
TOOLS

Proud owner of stock '76 CB750.

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[cdaiscool]

I am trying to get in the habit of turning mine off. I left it on for a week several times, and never had a puddle under it. Underneath the cover does smell like a bit of gas, but I figure that's gas evaporating off. Sounds like your float or float tubes are messed. Use brasso and clean up those brass tubes.

[seestheday]

Thorby, don't be scared of doing the carbs yourself.  I basically had zero motorcycle knowledge when I bought this bike, and a carb clean/rebuild was the 1st thing I did to it.

It did take me a long time to do though because I took my time (I did it over many nights during the winter).

So if you don't have time, that could certainly be a good reason to get them cleaned by someone else.

For me, it was an invaluable experience and opened up a lot of understanding to me about how my bike worked.