95 vortec won't run
 

I have a 95 vortec blazer 4.3 4x4 that was given to me it will start but shut off immediately waswondering what could be causing that. It starts up with prime runs rough for a couple seconds then shuts off..... it did sit for a couple years before i got it. just want to know where to start it seems to me that it is a fuel problem but after reading everything on here it could be a number of things........ also after reading some posts on here i checked the sensor after the cat and the wire was cut off where does it hook up