Its the fueling mate or a bad air leak it needs to be checked and adjusted, if you havn't got a afr guage/lambda of some sort you havn't got much chance if its as far out as it sounds!
tbh i would never change over a metering unit unless i had means to monitor/adjust the fueling as the unit could be setup completely wrong for you engine which does sound like the case at the moment.
As a last thought have you got a boost/vacuum gauge- does it hold a steady vacuum ?