Pilot error on fuel levels prior to take-off and one engine consuming excessive fuel.
NTSB crash report, page 3:
http://www.autopsyfiles.org/reports/FAA/AAR78-06.pdf
what I have heard, was the fuel gauges did not work, one of the reasons aerosmith pilots turned it down. the pilots had to literally use a stick, to measure fuel level before each flight. one of the engines was in poor condition,and to keep it running, pilots had to run it with rich mixuture of gas., causing flames to occasionaly blow out the back of the motor. pilots apparently did not take into consideration the high rate of fuel burned, due to this defective enginge, and they ran out of gas. paid for it with their lives.