> ...when it keeps dying... move the petc-o-c-k to
> the "PRI" postition ... does anyone know why

Fuel flow through the normal position is controlled by engine vacuum--the gate is held open via suction.  The PRIme route is a direct tap to the tank.

Btw, the vacuum petcock is a 'dumb down' feature that is supposed to eliminate the possibility of a fuel leak that can happen if a manual petcock is left ON and the float needle fails to seat.

If you have reoccurring feed problems that are fixed with PRI, you may have a vacuum leak.  Check the condition of the vacuum hose going to the petcock, and the petcock diaphragm.

Fuel starving can also be caused by a plugged vent at the gas cap.

Just as a hint....if for some reason you need to run all the gas out of the carb (and you don't want to drain the bowl with the screw).  You can turn the pet_c_o_c_k to ON, and pull the vacuum hose off on the right side of the carb.  Then just let the engine idle until it dies.  That way all the fuel in the line and carb is gone.