Aircraft Dynamics Help

I need some input from model developers with aircraft dynamics. I am working on a warbird model for FSXA and came up with in interesting problem. When I manually start the plane with the mixture control in cut position (I know it will not start this way) the plane will roll forward very fast, lift off, and flip over all very fast and with the parking brake on. If I apply just a small amount of mixture or use CTRL+E to start, everything works fine. Has anybody had experience with this before?Thanks :(