MIne started giving problems at about 45k.
But here is the thing, every once in a while while riding, reach down and turn off the stock valve. After a bit (depending on your speed) you will be able to verify that it is still shutting off the fuel.
On the flip side of this coin, if you are running hard and begining to feel as if it's starving for fuel, but the valve is on and reserve doesn't help, then ruleing out a vacume leak or the vent hose is not kinked. Then it's most likely the peckot diaphram.
The last indicator is seeing a drop of fuel from the vent orfice at the bottom of the valve it self.
hope this helps
