Topic: Odd Intermittent Fuel Leak

[Leanier]

I have been having a strange fuel leak issue with my '73 CB750.  I know one issue has to do with the float needles being dirty/ the floats improperly adjusted, and will be fixing that soon. 

However, every few days when the bike sits overnight, (or even sometimes just for a few hours) the bike will leak fuel out of the overflow tube on one carb.  usually one of the two internal carbs, but not always the same carb.

The strange part is it will do this even with my petcock in the off position.

I have tested the petcock off the carbs, and even with a mostly full tank it does not drip at all in the off position.

I am wondering if there is somehow pressure building up in the gas tank, and forcing gas past the petcock seal?  The tank never seems pressurized when I open up the lid, and I can hear air going past the gas cap lid vent sometimes.

The amount of gas that spills can be upwards of 1/2 gallon in an hour or so, so it isn't just a small leak or the fuel in the lines.

Any insight would be appreciated, sometime this week I will be doing a full carb rebuild and petcock rebuild as I don't know what else to do.

I have also had an odd issue where the petcock only feeds one of its two outlets, starving two carbs for gas but giving the other two plenty on lower gas amounts in the tank (but not close to reserve)  If thats related at all... (The petcock is totally clean inside, no buildup of crap at all..)

[2wheels]

It's time to rebuild the petcock.
I understand you tested it but if it leaks when shut off what else could it be?

[flybox1]

The strange part is it will do this even with my petcock in the off position.

+1 petcock leaking - rebuild

[75CBCafe]

For fuel to get out the overflow your float valves must be sticking / leaking by. I think you are on the right track to rebuild them. The only way for fuel to get into the carb is through these valves.

[Leanier]

Yea I do plan to rebuild everything, I just find it a little odd that the petcock won't leak when the tank is off the bike.  I have had the tank off sitting on the ground with the petcock off for a few days without any leaks. 
I mostly just want to make sure its not something odd so I don't go rebuild all of this to no avail.

[reddyvv]

You should check the strainer on the petcock. If it is torn it will allow rust particles or dirt to enter the petcock and you won't get a perfect seal every time.

[flybox1]

Tank OFF the bike and PC turned OFF....doesnt leak
TANK ON the bike and PC OFF...carbs leak 1/2 gallon an hour   

is the pressure of the tank on the ground pressing the PC up into the tank, maybe completing a seal or something?

it just doesnt make sense

how do you get your carb bowl to stop leaking? or does it leak no matter what?

check for cracks in the overflow standpipes.
check that the bowl drain screws are seated.

[Leanier]

For some reason it only leaks after sitting for a little while, and it doesn't leak all the time.  I can't find any pattern to when it leaks, seems to leak more often with a totally full tank.  But sometimes it'll go a week or two without doing it.  then like this morning, presto, a river of gas going down my garage...  I have no idea what it is.

the second I get the bike started and ride it it goes away... until the next random time.   its coming out of my overflow tube, and totally random so its not a consistant leak.  When I open the gas lid it doesn't sound pressureized, even when it's leaking.

I'm thinking it must be a combo of not seating floats and crap getting into my petcock somehow.   Only exlination for how random it is, and how much it is.

[flybox1]

well, if your PC is OFF, and working as it should, you shouldn't have leaks anywhere, even if your float valves are not working.
fix your PC first.
with your PC ON, and it leaks, then its your float height/float valves, or debris in your tank that need to be addressed.

[acollin]

Are you absolutely certain the leak is coming from the overflow tube and not simply finding its to the overflow tube then onto the ground.1/2 gallon is a lot of leakage to get through the fuel system and not see something obvious from the petcock.

Check to see if fuel residue is higher up on those two overflow hoses. If it is......

Good luck-- sorry for your trouble, but it is a good puzzle.

[tlbranth]

To leak 1/2 gallon it must be getting past the petcock. There ain't that much fuel in the carbs. You have dirt in the needle valves or sticking floats or fuel in the floats. One problem often overlooked is dimpling of the float tang where the valve touches it. It causes sporadic sticking open of the float valve. It can be fixed by filling the dimple with solder and filing flat or by filing the tang 'til the dimple's gone.