Possible causes for main engine not fired are as follows
1.If the Turning gear is engaged .It blocking valve prevents pilot air flowing to the pilot valve on the control stand .So Dis-engage the Turning gear before starting .
2.The shut off valve on the starting air cointains cracks or starting air pipe are closed .
3.The Automatic Starting air stop valve does not works on account of having been closed by hand ,either being jammed or its internal out let ducts being obstructed .
4.The Actuating valve for the Automatic starting air stop valve is jammed or opens only partially.
5.The NON RETURN VALVE in starting air pipe is struck and does not open.
6.Starting air pressure is too low , the Engine only oscillating .Flame baffle is obstructed.
7.The starting air control slide valves are either struck or the control air is not able to force them down on to the starting cam vent opening may be obstructed.
8.The starting valve do not open .
9.A working piston or any other running gear component prevents starting of the engine by reason of being seized or defective.
10.One or more starting control air pipes or control oil pipes are incorrectly connected or obstructed .
11.If the blocking device of the starting lever has been uncoupled by hand ,the engine can only be started if the reversing servomotor is closed to its end position .