You first need to check the pressure. No more then 3 PSI at the carb. It does not matter how you get the fuel there. Just 3 PSI is the Magic number. Of carouse a mechanical pump is not the recommended item with a weber. It pulsates with each hit of the cam lobe. The stock carb was built to take the pressure and the pulsation caused by the stock pump. Remember to keep the pump as close to the tank as you can. Or you will burn up the pump or have pick up problems. In tank insures you have a good fuel supply and it keeps the pump cool.