Steve, call me a pessimist, but I think there are two problems - 

(1)  For fuel to leak from the carb, it has to get there from the tank.
It must pass through the petcock, so yours must still be passing fuel. 

(2) The amount of leakage suggests that the float valve is good enough to prevent 
the carb from running rich / overflowing while running, but not good enough 
for when no fuel is being consumed.  It doesn't start overflowing straight away 
because it takes those couple of hours after a run for the very slow leak to over-fill 
the carb to the point where it overflows.  If the float valve is working correctly 
fuel should not overflow even if the petcock is on. 

Use a magnifying glass to check the float valve parts and its seal for tiny 
imperfections that prevent a 100% seal.


Cheers
Bernard  (NZ)