340 Starting issue

I have a 73 Duster with a 340. The motor has been completely rebuilt with everything being new. All engine components are new. New gas tank, fuel pump, rebuilt carb. Was having some gas supply issue with the mechanical fuel pump so I put an electric fuel pump inline and it took care of the issue. I took it out for a test drive and discovered a power steering leak. I tightened up the lines and had the car running until it quit. Now it will only run for a second or two then quits. I put a brand new carb on it and still it will only run for a few seconds. I can't figure this one out especially since it was running.