72' Mustang Raw exhaust fumes

My 72' Mustant has raw exhaust fumes. The smell is so strong I have to drive with my windows up. I purchased the car 2 years ago, so I am not sure what modifications the previous owners did. I came with a holley 4 barrel intake manifold. I installed a 600 cfm Edelbrock carb and an ignition box, but it did not help. The strange thing is that when I try to lower the jets on the carb just a bit, it sputters and turns off. How is it possible if it is getting the minimum amout of gas, that it has raw exhaust fumes.
Please help.