I'm glad you like it, thanks.
First, don't mess with the sdef files. They are where they need to be in order for the sounds to blend properly, which probably explains why you can't hear the spark plugs now. Again, DO NOT change SDEF files or .wav files--they're all custom made.
Second, The sound you're hearing during start up is a .wav file of an actual Huey starting up, it's not a synthesized fake sound that spools up and spools down based on you manipulating the start trigger. That's why you hear the sound abruptly stop when you let off the trigger, which you would never do in real life anyway unless you had a hot start. That's just a limitation of the game I have no control over.