Engines with rusty bearings don't turn smoothly, there is resistance not related to compression, and the movement often feels jerky or "crunchy".
Carefull here, I have had "crunchy" bearings from congealed castor oil. Run some fuel though it and it goes away. Not that rust can't cause this, just not the only reason. Also the bearings can get a light film of rust that you can't feel, the rust wears off when you run it, eventually the bearings will whine from the wearing off of rust which causes the balls to be out of round.