[skyomish]

The I-153 and a few other planes with float carbs and not fuel injection with cut out if you pull negative G's. Watch it when dropping your nose in a hurry.

I do think this is the problem. If you pull Neg-G's the engine will STOP DEAD.
Try not to do a NOSE over but roll your plane to dive.

Depalmer hit it dead on - read his post.