If the engine fires off time, the starter will jump off the flywheel and do like you describe. (If it has NO LOAD, i.e. no plugs in it, the same thing will happen.) It is normal. However, you say you are trying to start it, so the plugs are in....
So, you had the flywheel off...first thing to check is make absolutely sure you have the flywheel back on and aligned with the key correctly. It is kinda hard to do with the magnets pulling on it. Look from the top to align as you lower the wheel. The flat of the key should be parallel to the centerline of the crankshaft. It most likely is parallel to the taper of the crankshaft right now. If not lined up with the centerline of the crank, make sure it is, and it will be a bit easier to put on. The requirement I just described regarding the key alignment is in every factory manual I have ever looked at. Then make sure the nut is torqued to 140 ft-lbs, cause if it pops, it could shear the flywheel key immediately.
Then, if the flywheel checks out, look at the orange wires from the pack to the coils. Make absolutely sure the orange/blue wires go to the top coils, the orange wires go to the middle coils, and the orange/green wires go to the bottom coils. Don't cross from side to side....(even tho the wires are not long enough to do that....) STBD wire to STBD coils, PORT wires to PORT coils.
Let us know what you find.