Fuel issue
 

I've removed a leaking rear tank on my '93 F-250, 7.3L diesel. Tank selector switched to front, but truck began running rough and smoking. I've since plugged both fuel lines to/from rear tank, but haven't had a positive change in performance. Thought I might run out any air that might be caught in the system, but not panning out.
Is there a step or trick I'm missing? Have never had an issue running the front tank before.