First of all the NC50 was not meant to push start.
It aint gonna happen.
What I think is happening is that you are using the gears only to tighten up the spring that is meant to lift the tap start lever up a couple inches after you pull the brake handle (or however you have it rigged up. YOu only want enough power in that spring to lift that arm. try releasing it with your rigged up system. prolly won't do it..
Now try BACKing your ped up for the same distance that you pushed it. That should release the tension to a reasonable level if my guess is right.
This is certainly an interesting problem.
If you try to take that left cover off again while that spring in under high tension it could cause major damage to you.
Is your gasket still good for the left side cover? if not you could loose all your tranny oil that way.
Let us know