Petcock diaphragm O-ring

[LoHo]

I put a new kit into my leaky petcock, the first time I have ever taken it apart, and was wondering: If one replaced the O-ring of the little vacuum-controlled piston, would that not stop the leaking? Is that not the only path for the fuel? To my primitive engineering knowledge, it looks like that is the key and that repairing that ring would solve the leak if everything else were intact.

[CaptonZap]

I put a new kit into my leaky petcock, the first time I have ever taken it apart, and was wondering: If one replaced the O-ring of the little vacuum-controlled piston, would that not stop the leaking? Is that not the only path for the fuel? To my primitive engineering knowledge, it looks like that is the key and that repairing that ring would solve the leak if everything else were intact.

That it is. The other part of the equation is the seat that the o-ring seals against. Make sure it is smooth and free of burrs.
I don't know what durometer the original O-rings were, or if it would make a difference in function, but at the cost of an o-ring and some time, I would sure give it a try.
The spring is what gives it, the O-ring, the sealing pressure, so if you stretch the spring a little, and you rig a gas can with a three or four foot head, and it didn't leak, I would consider it fixed. The most head it will see on the bike is a little over a foot, depending on what gas tank you have, and how full you fill it.
Let us know what you find out. CZ

[skids]

Don't stretch that spring much because at some rpms there isn't really much vacuum pressure to keep the valve open... You can see that when you sunch the carbs if you open the throttle slowly. A lot of that depends on your exhaust system and probably your carb filter choices.