Leaky Petcock

Good morning all.

I have a strange situation. I have out several new petcocks(and I mean new) on my 1975 Z1 and all of them leak. The last one I got I took the tank off the bike, set it on the bench with the new petcock and it didn't leak. I left it there for a few days to make sure. I put it on the bike, ran it up and down the street and parked it. The next day I came out and it was leaking.

I can tell it is leaking because there is gas coming out of the over flow on the carbs.

Is it possible for there to be a vacuum in the bike that is drawing the gas through the petcock?

When I ran it the other day up and down the street I notice when I got off the bike that there was a sucking sound coming from the gas cap. When I opened it up I heard a sucking sound. There was a vacuum in the tank.

The gas coming out of the carbs is another issue. I have cleaned the carbs several times and adjusted the floats to sit at 22mm. The carbs were rebuilt a couple years ago and have all new jets and needles.

I am getting frustrated with the bike because I can't get it to run right. It only has 14000 on it. I love this bike and have been worn
King on it for years. Kinda stuck right now.

What else do you need to know?

Since you heard a sucking sound from the tank cap and found there was a vacuum in the tank it would appear that your tank cap vent is not working. I would take the cap apart and clean the vent and make sure it works well, then see if the leak stops.

Here's my theory -

Assume for a minute that the tank is sealed tight because of a clogged vent. You ride the bike and everything seems to be ok. Now you park the bike. The heat from the engine warms up the fuel and air in the tank. As the air heats it expands and raises the air pressure in the tank which in turn could apply enough pressure on top of the fuel to force fuel down through the petcock (even if the petcock is good) and through the float valves causing an overflow condition until the pressure is relieved in the tank.

Again, it's just a theory, but if you get the tank vent working properly it might fix the problem. Another way to test the theory is to leave the tank cap open when you park the bike and see if the petcock still leaks. Ed

Thanks. I cleaned the vent in the cap and rode it around the block. Now I will just wait and see.

davej6997 wrote: ... gas coming out of the over flow on the carbs...have cleaned the carbs several times and adjusted the floats to sit at 22mm. The carbs were rebuilt a couple years ago and have all new jets and needles....

Float valves are leaking for whatever reason.
Measured float height isn't the critical factor.
Fuel level being within specs is the critical factor.

Whatever float height results in correct fuel level is the correct float height, regardless of dry-measured float height.

Clear tube test is method to determine fuel level.

Good Fortune!

I cleaned the tank vent and the gas has stopped leaking. I will work on adjusting the floats next. Thanks guys.