There is a rubber O-ring that seals the petcock to the tank. (PN 51039)
Could be that has wore out or developed a crack.
There are a couple other O-rings that could be the problem.
Is it leaking between the tank & petcock, or more from the petcock itself?
If or when mine starts leaking, I'll replace it with an on/off/res petcock. I've put them on both my KZ's and much prefer them. Just have to cap off the vacuum to the carbs. Plus, they are around $26 from z1enterprises.com.
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