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Engine wont turn over to start ?
Most commonly caused by an interlock being active. Interlocks which may be fitted to you engine may include but aren’t limited to:- Turning gear engaged
- Protective shield not in place around flywheel
- Engine/Gearbox shutdown still active
- Throttle or propeller pitch not at zero position (for engine driven propeller shafts and controllable pitch propellers)
- Shaft brake engaged
If the engine is started by compressed air then the problem may be that there is insufficient air pressure available, check the air receivers. If the pressure is ok there may be an isolation valve between the receiver and the engine closed. If the air starting is via an air driven starter motor, control solenoid or the motor itself may be faulty. If the engine is started by direct air starting in the cylinders then there may be a fault with the air distributer, maybe it isn’t receiving pilot air due to a restriction or blockage. This could happen if water has been allowed to build up in the air system and mix with oil carried over from the compressor. From my experience, distributers rarely fail and the problem has been that the engine has been sitting in a dead spot. By this I mean that the engine has stopped between two cylinders on the distributer. This isn’t supposed to happen, the engines are supposed to be able to be started from any position but in reality and my past experience that is not the case. The answer is to put the turning gear back in and turn the engine over a little (don’t forget to take the gear out when you are finished)If the engine is electrically started by an electrical starter motor the problem may be that the battery voltage is not sufficient or that the battery isolation switch is in the off position. If the battery is fully charged, connected and not isolated it would suggest that the problem lies with the starter motor. Always check that the electrical connections are secure and there is a good earth. On a number of occasions I have witnessed suspected starter motor failure when the problem has been down to bad earthing. This is particularly common on tenders and motor cars where corrosion is present. If the motor does not make any movement or sound when trying to start, it would suggest that the solenoid is sticking or has failed. Sometimes a tap with a hammer can be enough to free it and allow the solenoid to engage but once this starts happening the solenoid is on its way out, get a new one. If the motor clicks and it sounds like the motor is engaging with the flywheel but not turning then the fault may be with the contacts inside the motor which are closed when the pinion fully extends allowing current to be supplied to the motors windings. If the contacts are closing the windings themselves may have failed. 
