Gas tank pressure and overflowing carbs?

[Port]

I have been chasing an issue where my carbs are leaking out of the pilot air jet on the rear of the carbs. The carbs are clean, recently rebuilt. When i fist hook them up to fuel, they fill to the right height. if i leave them for an hour or so with the petcock open and the tank about 2ft higher than the carbs one of the carbs will leek out of the pilot air jet and if i check the fuel height then, it will be about 10mm higher. yesterday it was the right carb, i checked and cleaned off the float valve seat. checked it again today and now the left carb is doing it.

i lost the spring to my vacuum petcock so it is always open, i am replacing it with non-vacuum one whenever it gets here. To drain the carbs i have to tilt the tank back and prop it up so that i can unhook the fuel line form the petcock. when i do that, gas shoots out the petcock once the hose pops off as it seems to be pressurized.

Is the tank supposed to have positive pressure? I know that tanks have vents but i was under the impression they were to allow air into the tank as fuel drained.

Is this positive pressure causing my overflowing carbs?

Will the manual petcock solve this issue or do i have a venting/valve issue that i need to solve?

 

[gggGary]

See what happens if you leave the cap loose. The stock fuel cap is vented but it's often plugged. If the rubber seal is good (rare LOL) you might be pressurizing the tank.

 

[XSLeo]

Yes tanks can pressurize if the vent is plugged. On my 75 if I let it set in the sun with pet cocks on the carbs would overflow. Cleaning the vent in the gas cap fixed that.
In your case it could be the tank height above the carbs. This extra height increases pressure at the float valve. Float valves are not designed to be fuel shuts offs. they are designed to be fuel metering devices. They keep the fuel level in the carbs when the engine is running. The petcock is the fuel shut off.
Some say you can shorten the lever on the vacuum pet cock enough so you can turn the lever straight up and have a fuel shut off position. I don't know if this works, never tried it.
For what you are doing now I might suggest getting a lawn mower shut off valve. They are black plastic with a red valve. You can get them most anywhere lawn mower parts are sold. Most auto parts stores have a lawn mower section. Wal Mart. TSC.
Thought I had a pic but can't find it.
Leo