I'm glad you figured it out.
The way it should work is you hit the eng start switch, let the engine spool, push the throttle forward into the idle position. That's it.
If you are having to press the throttle cutoff to being spooling the engine during start, your real throttle was not at idle when you loaded into the 3D world. Which the sim will realize and move your throttle forward, into the idle stop.
The effect of this is that you will need to place your throttle at idle, press the throttle cutoff, which places the throttle at cutoff again. allowing you to start the engine.
I'm telling you, take some time in your pit and play with the throttle to understand how this works. It is incorrect for you to have to move the cutoff lever during the start.