If someone put a vent in the backplate, it was to run a pump or pressurize the fuel tank. It will not contribute to the engine's running if it's left open to the atmosphere. You'll wind up with a lower power level of the engine because its pumping action will be reduced, and a tendency for the engine to run lean because it will be sucking additional air into the crankcase. If it was running a pump or pressurizing a fuel tank, the lower-end of the engine is essentially sealed by the pump or tank....that works, but venting to the atmosphere doesn't.