Out of the blue, the aircraft in my library aren't turned on when I choose one to fly. I checked my throttle and it's calibrated. I have a button on my throttle designated to turn on engines. It starts the engines but they don't stay engaged and eventually shut off. Is there a button on the flight simulator settings to have the engines already engaged when you select one to fly?
have you tried the Strg + e to start? this is the standard cold start key combination full stop. you need to look at the sidings connected to your throttle very closely. When you are starting an engine there are more steps than simply starting. If you look at the instrument panel of your aircraft you will see that you need to turn on the power and the generator and then you need to turn the key to the start position and only then will your engine indeed stay running. So again go back and look at the settings connected with your install throttle as you see them in FSX and then see if that doesn't help. Feel free to contact me again if you can't get it going.