Vacuum pulse line for petcock

The pulse line does not supply constant vacume, it pulses from positive to negative as the piston cycles. The only other source of vacume is the venturi port on the carb. 

And it should be enough to open the petcock?

The venturi vacume will probably open the petcock once the engine starts. There will be no fuel flow until it does. I don't know what your petcock requirements are. I personally would keep it simple with a manual petcock. 

I agree,

  keep it simple, just turn off the fuel manually. The "automatic" system you envision will likely introduce LESS reliability and more options for failure than it cures.

Thanks. The engine isn't used in a plane so its not very critical if it shuts off....and the convenience of auto full shut off is well worth it