L2350 700 hrs. Grandson ran fuel dry. I filled the tank, changed the fuel filter, bled the filter, injector pump and the three injector lines. Upon starting, tractor barely stayed running. Ran engine and loosened each of the injector lines. From this determined that only center cylinder was firing. Probability that two glow plugs failed simultaneously is low. Could the injectors be plugged by crud possibly caused when fuel was sucked dry?