The vacuum operated petcock in not a safety item, it was put there for EPA reasons, it prevents the carb bowl from refilling during fuel evaporation to atmosphere during extended periods of inoperation. That is why the overflow on some bikes go to a carbon canister just like your automobile intake manifold, gas tank vent to one.

Shutting the petcock to off should not be the last thing when shutting down the engine, it should be the first action in anticipation to shutting down the engine, thus running down the level of fuel in the carb. This will keep any fuel from escaping into the manifold or into the atmosphere. Also once the bike is stopped and engine stopped the residual heat from the cylinders will boil out the fuel from the carb, so the less there, the less to expand and overflow or vent. This action will also stop the flooding if a restart in immediate.

 

Do not disagree with what you are saying. And a great idea. However, the manual petcock does all that. What the EPA told Harley was most operators were not using it. So they put a fail safe there for safety.

 

Put a manual petcock in there and be done with your problems. It don't take long to get into the habit of turning it on and off. Becomes part or your normal start up or shut down routine. YD

 

sustained wot can cause the petcock to pinch off and cause a possible lean out.
as the plate opens, it becomes nearer to atmospheric pressure a transition to a positive. will fuel still flow, why sure because the cyl's are pumps ant there fore move air and as mr. dan stated, this air flow through the venturi causes a drop even though positive and that pulls up the fuel.
i blanked mine off due to this because i do push it on occasions.