Cranks but no start

I bought this car a few months back and have replaced the head gasket, intake gasket, starter, mass airflow sensor, Distributor, cap and rotor, oxygen sensor, valve cover gasket, fuel pump, fuel filter, spark plugs, and a few other ignition parts.
Problem is that it will not start now. It ran but ran poorly after the head gasket replacement and then after about fifty miles of very poor performance. It just puttered out and shut off and refuses to start. I have gotten it to start a few times but it only ran for about fifteen seconds and shuts off and if you press the accelerator at all it does even faster.
After it starts like that I pull the spark plugs and they are coated black and smell like gas.
I am at the end of my knowledge and would appreciate any help!